diff --git a/Day_31_to_40/Day_40/README.md b/Day_31_to_40/Day_40/README.md
index 716b0d3..57f6142 100644
--- a/Day_31_to_40/Day_40/README.md
+++ b/Day_31_to_40/Day_40/README.md
@@ -1,2 +1,57 @@
 # Things I learned in: Day_31_to_40/Day_40
 **Note:** use the github provided TOC for navigaing.
+
+
+
+
+# qsn schedul
+1. complete linkedlist theory [x]
+2. Creating a Linked List[x]
+3. Insertion in the Middle[x]
+4. Recursive Reverse a Linked List
+5. Recursively Reverse a Linked List Code
+6. Iterative Reverse
+7. Iteratively Reverse a Linked List Code
+8. K-Reverse a Linked List
+9. operator overloading[x]
+additional
+1. string generation acode recursion 
+2. tower of honoi 
+3. advanced backtracking webinar
+
+
+# Oracal plane:
+1. do most of the problems from fraz sheet
+2. complete DSA and OOPs
+    + cover stack
+    + cover heap
+    + cover BT and BST
+    + Cover hash table
+    + cover oops
+    + sorting
+    + binary search problems
+    + bitmask
+    + number theory
+    + 2d array problems
+    + Dynamic memory allocation
+    + cover DP
+    + cover graph
+3. practice on codeforces for time management
+    + do some ad hoc
+4. Practice Apti
+    + cover probability
+    + cover loss and profit
+    + cover men and work
+5. practice SQL
+6. Prepare for ML interviews
+    - Do statistics
+    - Revise previously done ML and DL NLP parts
+    - Cover Unsupervised Algo
+    - prepare a bit for tf and pytorch
+    - cover PCA
+    - cover EDA
+    - cover come of the ML interview questions (part 2)
+7. Do some Excel and Power Bi
+8. Practice python numpy,pandas problems
+
+
diff --git a/Day_31_to_40/Day_40/add_operator_overloading.cpp b/Day_31_to_40/Day_40/add_operator_overloading.cpp
new file mode 100644
index 0000000..de585ef
--- /dev/null
+++ b/Day_31_to_40/Day_40/add_operator_overloading.cpp
@@ -0,0 +1,118 @@
+// https://youtu.be/8ymiMHQPgZY
+// https://github.com/darkprinx/ACM-SOLUTION/blob/master/Spoj%20Solutions/FILRTEST%20-%20File%20Recover%20Testing.cpp
+#include<bits/stdc++.h>
+
+using namespace std;
+
+// void _print(ll t) {cerr << t;}
+// void _print(int t) {cerr << t;}
+// void _print(string t) {cerr << t;}
+// void _print(char t) {cerr << t;}
+// void _print(lld t) {cerr << t;}
+// void _print(double t) {cerr << t;}
+// void _print(ull t) {cerr << t;}
+
+// template <class T, class V> void _print(pair <T, V> p);
+// template <class T> void _print(vector <T> v);
+// template <class T> void _print(set <T> v);
+// template <class T, class V> void _print(map <T, V> v);
+// template <class T> void _print(multiset <T> v);
+// template <class T, class V> void _print(pair <T, V> p) {cerr << "{"; _print(p.ff); cerr << ","; _print(p.ss); cerr << "}";}
+// template <class T> void _print(vector <T> v) {cerr << "[ "; for (T i : v) {_print(i); cerr << " ";} cerr << "]";}
+// template <class T> void _print(set <T> v) {cerr << "[ "; for (T i : v) {_print(i); cerr << " ";} cerr << "]";}
+// template <class T> void _print(multiset <T> v) {cerr << "[ "; for (T i : v) {_print(i); cerr << " ";} cerr << "]";}
+// template <class T, class V> void _print(map <T, V> v) {cerr << "[ "; for (auto i : v) {_print(i); cerr << " ";} cerr << "]";}
+
+#ifndef ONLINE_JUDGE
+#define debug(x) cerr << #x <<" "; _print(x); cerr << endl;
+#else
+#define debug(x)
+#endif
+
+
+#define fastio() ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
+#define MOD 1000000007
+#define MOD1 998244353
+#define INF 1e18
+#define nline "\n"
+#define pb push_back
+#define ppb pop_back
+#define mp make_pair
+#define ff first
+#define ss second
+#define PI 3.141592653589793238462
+#define set_bits __builtin_popcountll
+#define sz(x) ((int)(x).size())
+#define all(x) (x).begin(), (x).end()
+#define my_sizeof(type) ((char *)(&type+1)-(char*)(&type))
+#define print_vec(x) for(auto i:x) cout<<i<<" "; // prints elements of an int vector
+#define check_all_zero(v) all_of(v.begin(), v.end(), [](int i) { return i==0; })
+#define str_to_int(x) stoi(x) // converts a string to an int
+#define eps 1e-9
+#define gcd(x,y) __gcd(x,y)
+#define lcm(a,b) (a*(b/gcd(a,b)))
+#define maxall(v) *max_element(all(v))
+#define minall(v) *min_element(all(v))
+#define max3(a,b,c) max(a,max(b,c))
+#define min3(a,b,c) min(a,min(b,c))
+
+
+typedef long long ll;
+typedef long long int lli;
+typedef unsigned long long ull;
+typedef long double lld;
+typedef vector<int> vi;
+typedef vector<ll> vll;
+typedef pair<int, int> pii;
+typedef pair<ll, ll> pll;
+typedef pair<ll, bool> pib;
+typedef pair<double, double> pdd;
+typedef pair<int, double> pid;
+typedef pair< int, pii> ipii;
+// typedef tree<pair<int, int>, null_type, less<pair<int, int>>, rb_tree_tag, tree_order_statistics_node_update > pbds; // find_by_order, order_of_key
+// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Code Below ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~//
+// https://youtu.be/BO2KagRMS3M
+class Complex{
+private:
+    int real,imag;
+public:
+    // default constructor
+    Complex(){
+        real = 0;
+        imag = 0;
+    }
+    // user defined constructor
+    Complex(int r,int i){
+        real = r;
+        imag = i;
+    }
+    void print(){
+        cout<< real<<"+"<<imag<<"i"<<nline;
+    }
+
+    Complex operator +(Complex c){
+        Complex temp;
+        temp.real = real + c.real;
+        temp.imag = imag + c.imag;
+
+        return temp;
+    }
+};
+
+
+int main() {
+#ifndef ONLINE_JUDGE
+    freopen("errorf.in", "w", stderr);
+#endif
+
+    fastio();
+    Complex c1(2,3);
+    Complex c2(34,5);
+    Complex c3(12,4);
+    Complex c4;
+
+    // c4 = c1+c2; is equivalent to c1.add(c2)
+    c4 = c1+c2+c3;
+
+    c4.print();
+}
diff --git a/Day_31_to_40/Day_40/add_operator_overloading.exe b/Day_31_to_40/Day_40/add_operator_overloading.exe
new file mode 100644
index 0000000..dd0d958
Binary files /dev/null and b/Day_31_to_40/Day_40/add_operator_overloading.exe differ
diff --git a/Day_31_to_40/Day_40/cin_cout_overloaded_for_ll.cpp b/Day_31_to_40/Day_40/cin_cout_overloaded_for_ll.cpp
new file mode 100644
index 0000000..e1228ac
--- /dev/null
+++ b/Day_31_to_40/Day_40/cin_cout_overloaded_for_ll.cpp
@@ -0,0 +1,164 @@
+// https://youtu.be/8ymiMHQPgZY
+// https://github.com/darkprinx/ACM-SOLUTION/blob/master/Spoj%20Solutions/FILRTEST%20-%20File%20Recover%20Testing.cpp
+#include<bits/stdc++.h>
+
+using namespace std;
+
+// void _print(ll t) {cerr << t;}
+// void _print(int t) {cerr << t;}
+// void _print(string t) {cerr << t;}
+// void _print(char t) {cerr << t;}
+// void _print(lld t) {cerr << t;}
+// void _print(double t) {cerr << t;}
+// void _print(ull t) {cerr << t;}
+
+// template <class T, class V> void _print(pair <T, V> p);
+// template <class T> void _print(vector <T> v);
+// template <class T> void _print(set <T> v);
+// template <class T, class V> void _print(map <T, V> v);
+// template <class T> void _print(multiset <T> v);
+// template <class T, class V> void _print(pair <T, V> p) {cerr << "{"; _print(p.ff); cerr << ","; _print(p.ss); cerr << "}";}
+// template <class T> void _print(vector <T> v) {cerr << "[ "; for (T i : v) {_print(i); cerr << " ";} cerr << "]";}
+// template <class T> void _print(set <T> v) {cerr << "[ "; for (T i : v) {_print(i); cerr << " ";} cerr << "]";}
+// template <class T> void _print(multiset <T> v) {cerr << "[ "; for (T i : v) {_print(i); cerr << " ";} cerr << "]";}
+// template <class T, class V> void _print(map <T, V> v) {cerr << "[ "; for (auto i : v) {_print(i); cerr << " ";} cerr << "]";}
+
+#ifndef ONLINE_JUDGE
+#define debug(x) cerr << #x <<" "; _print(x); cerr << endl;
+#else
+#define debug(x)
+#endif
+
+
+#define fastio() ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
+#define MOD 1000000007
+#define MOD1 998244353
+#define INF 1e18
+#define nline "\n"
+#define pb push_back
+#define ppb pop_back
+#define mp make_pair
+#define ff first
+#define ss second
+#define PI 3.141592653589793238462
+#define set_bits __builtin_popcountll
+#define sz(x) ((int)(x).size())
+#define all(x) (x).begin(), (x).end()
+#define my_sizeof(type) ((char *)(&type+1)-(char*)(&type))
+#define print_vec(x) for(auto i:x) cout<<i<<" "; // prints elements of an int vector
+#define check_all_zero(v) all_of(v.begin(), v.end(), [](int i) { return i==0; })
+#define str_to_int(x) stoi(x) // converts a string to an int
+#define eps 1e-9
+#define gcd(x,y) __gcd(x,y)
+#define lcm(a,b) (a*(b/gcd(a,b)))
+#define maxall(v) *max_element(all(v))
+#define minall(v) *min_element(all(v))
+#define max3(a,b,c) max(a,max(b,c))
+#define min3(a,b,c) min(a,min(b,c))
+
+
+typedef long long ll;
+typedef long long int lli;
+typedef unsigned long long ull;
+typedef long double lld;
+typedef vector<int> vi;
+typedef vector<ll> vll;
+typedef pair<int, int> pii;
+typedef pair<ll, ll> pll;
+typedef pair<ll, bool> pib;
+typedef pair<double, double> pdd;
+typedef pair<int, double> pid;
+typedef pair< int, pii> ipii;
+// typedef tree<pair<int, int>, null_type, less<pair<int, int>>, rb_tree_tag, tree_order_statistics_node_update > pbds; // find_by_order, order_of_key
+// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Code Below ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~//
+
+class node{
+public:
+    int data;
+    node*next;
+
+    node(int d){
+        data = d;
+        next = NULL;
+    }
+    ~node(){
+        if(next!=NULL){
+            delete next;
+        }
+    }
+};
+
+void print_ll(node* head){   // at the print time we dont pass by reference
+    while(head!=NULL){
+        cout<<head->data<<", ";
+        head = head->next;
+    }
+    cout<<endl;
+}
+
+void insert_head(node*& head,int d){    //passing by reference
+    if(head == NULL){
+        head = new node(d);             // creating node in dynamic memory
+                                        // which only returns the address of
+                                        // created node
+        return;
+    }
+
+    node *n = new node(d);
+    n->next = head;
+    head= n;
+}
+
+node* take_input(){
+    int d;
+    cin>>d;                     // taking the data
+    node* head = NULL;          // creating the head pointer
+
+    while(d!=-1){              // - + untill user provides -1 as the input we ill keep taking input from user
+        insert_head(head,d);   //   | and insert in the head(we can use other insert operations too)
+        cin>>d;                //   | and take input from the user, which will eventually update d.
+    }                          // - +
+    return head;               // we return head as we are not taking head in the function input.
+}
+
+
+node* take_input_txt(){
+    int d;
+    node* head = NULL;
+
+    cin>>d;
+    while(d!=-1){
+        insert_head(head,d);
+        cin>>d;
+    }
+
+    return head;
+}
+
+
+ostream& operator<<(ostream &os,node* head){
+    print_ll(head);
+    return os;
+}
+
+istream& operator>>(istream &is, node*&head){
+    head = take_input_txt();
+    return is;
+}
+
+
+int main() {
+#ifndef ONLINE_JUDGE
+    freopen("errorf.in", "w", stderr);
+#endif
+
+    fastio();
+    node*head;
+    node*head2;
+
+    cin>>head;
+    cin>>head2;
+    cout<<head;
+    // cout<<nline;
+    cout<<head2;
+}
diff --git a/Day_31_to_40/Day_40/cin_cout_overloaded_for_ll.exe b/Day_31_to_40/Day_40/cin_cout_overloaded_for_ll.exe
new file mode 100644
index 0000000..b204674
Binary files /dev/null and b/Day_31_to_40/Day_40/cin_cout_overloaded_for_ll.exe differ
diff --git a/Day_31_to_40/Day_40/errorf.in b/Day_31_to_40/Day_40/errorf.in
new file mode 100644
index 0000000..e69de29
diff --git a/Day_31_to_40/Day_40/inputf.in b/Day_31_to_40/Day_40/inputf.in
index 7ed6ff8..df669df 100644
--- a/Day_31_to_40/Day_40/inputf.in
+++ b/Day_31_to_40/Day_40/inputf.in
@@ -1 +1 @@
-5
+1 2 3 -1
\ No newline at end of file
diff --git a/Day_31_to_40/Day_40/kth_node_from_then_end.cpp b/Day_31_to_40/Day_40/kth_node_from_then_end.cpp
new file mode 100644
index 0000000..d1130f6
--- /dev/null
+++ b/Day_31_to_40/Day_40/kth_node_from_then_end.cpp
@@ -0,0 +1,217 @@
+// https://youtu.be/8ymiMHQPgZY
+// https://github.com/darkprinx/ACM-SOLUTION/blob/master/Spoj%20Solutions/FILRTEST%20-%20File%20Recover%20Testing.cpp
+#include<bits/stdc++.h>
+
+using namespace std;
+
+// void _print(ll t) {cerr << t;}
+// void _print(int t) {cerr << t;}
+// void _print(string t) {cerr << t;}
+// void _print(char t) {cerr << t;}
+// void _print(lld t) {cerr << t;}
+// void _print(double t) {cerr << t;}
+// void _print(ull t) {cerr << t;}
+
+// template <class T, class V> void _print(pair <T, V> p);
+// template <class T> void _print(vector <T> v);
+// template <class T> void _print(set <T> v);
+// template <class T, class V> void _print(map <T, V> v);
+// template <class T> void _print(multiset <T> v);
+// template <class T, class V> void _print(pair <T, V> p) {cerr << "{"; _print(p.ff); cerr << ","; _print(p.ss); cerr << "}";}
+// template <class T> void _print(vector <T> v) {cerr << "[ "; for (T i : v) {_print(i); cerr << " ";} cerr << "]";}
+// template <class T> void _print(set <T> v) {cerr << "[ "; for (T i : v) {_print(i); cerr << " ";} cerr << "]";}
+// template <class T> void _print(multiset <T> v) {cerr << "[ "; for (T i : v) {_print(i); cerr << " ";} cerr << "]";}
+// template <class T, class V> void _print(map <T, V> v) {cerr << "[ "; for (auto i : v) {_print(i); cerr << " ";} cerr << "]";}
+
+#ifndef ONLINE_JUDGE
+#define debug(x) cerr << #x <<" "; _print(x); cerr << endl;
+#else
+#define debug(x)
+#endif
+
+
+#define fastio() ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
+#define MOD 1000000007
+#define MOD1 998244353
+#define INF 1e18
+#define nline "\n"
+#define pb push_back
+#define ppb pop_back
+#define mp make_pair
+#define ff first
+#define ss second
+#define PI 3.141592653589793238462
+#define set_bits __builtin_popcountll
+#define sz(x) ((int)(x).size())
+#define all(x) (x).begin(), (x).end()
+#define my_sizeof(type) ((char *)(&type+1)-(char*)(&type))
+#define print_vec(x) for(auto i:x) cout<<i<<" "; // prints elements of an int vector
+#define check_all_zero(v) all_of(v.begin(), v.end(), [](int i) { return i==0; })
+#define str_to_int(x) stoi(x) // converts a string to an int
+#define eps 1e-9
+#define gcd(x,y) __gcd(x,y)
+#define lcm(a,b) (a*(b/gcd(a,b)))
+#define maxall(v) *max_element(all(v))
+#define minall(v) *min_element(all(v))
+#define max3(a,b,c) max(a,max(b,c))
+#define min3(a,b,c) min(a,min(b,c))
+
+
+typedef long long ll;
+typedef long long int lli;
+typedef unsigned long long ull;
+typedef long double lld;
+typedef vector<int> vi;
+typedef vector<ll> vll;
+typedef pair<int, int> pii;
+typedef pair<ll, ll> pll;
+typedef pair<ll, bool> pib;
+typedef pair<double, double> pdd;
+typedef pair<int, double> pid;
+typedef pair< int, pii> ipii;
+// typedef tree<pair<int, int>, null_type, less<pair<int, int>>, rb_tree_tag, tree_order_statistics_node_update > pbds; // find_by_order, order_of_key
+// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Code Below ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~//
+
+class node{
+public:
+    int data;
+    node*next;
+
+    node(int d){
+        data = d;
+        next = NULL;
+    }
+    ~node(){
+        if(next!=NULL){
+            delete next;
+        }
+    }
+};
+
+void print_ll(node* head){   // at the print time we dont pass by reference
+    while(head!=NULL){
+        cout<<head->data<<", ";
+        head = head->next;
+    }
+    cout<<endl;
+}
+
+void insert_head(node*& head,int d){    //passing by reference
+    if(head == NULL){
+        head = new node(d);             // creating node in dynamic memory
+                                        // which only returns the address of
+                                        // created node
+        return;
+    }
+
+    node *n = new node(d);
+    n->next = head;
+    head= n;
+}
+
+node* take_input(){
+    int d;
+    cin>>d;                     // taking the data
+    node* head = NULL;          // creating the head pointer
+
+    while(d!=-1){              // - + untill user provides -1 as the input we ill keep taking input from user
+        insert_head(head,d);   //   | and insert in the head(we can use other insert operations too)
+        cin>>d;                //   | and take input from the user, which will eventually update d.
+    }                          // - +
+    return head;               // we return head as we are not taking head in the function input.
+}
+
+
+node* take_input_txt(){
+    int d;
+    node* head = NULL;
+
+    cin>>d;
+    while(d!=-1){
+        insert_head(head,d);
+        cin>>d;
+    }
+
+    return head;
+}
+
+
+ostream& operator<<(ostream &os,node* head){
+    print_ll(head);
+    return os;
+}
+
+istream& operator>>(istream &is, node*&head){
+    head = take_input_txt();
+    return is;
+}
+
+
+// reverse Linked List using recursion
+void reverse_ll(node* &head){
+    // think this is a ll: 1->2->3->NULL
+    node* prev = NULL;
+    node* c = head;
+    node* n;
+    while(c !=NULL){
+        n = c->next;  // now n is pointing to 2
+        c->next = prev;
+        prev = c;
+        c =n;
+    }
+    head = prev;
+    return;
+}
+
+node* reverse_rec(node* &head){
+    if(head=NULL or head->next == NULL){
+        return head;
+    }
+
+    // do recurcive case
+    node* shead = reverse_rec(head->next);
+    node* temp = shead;
+    while(temp->next!=NULL){
+        temp = temp->next;
+    }
+    head->next = NULL;
+    temp->next = head;
+
+    return shead;
+}
+
+// https://ide.codingblocks.com/s/610493
+node* kth_element(node*&head,int k)
+{
+    node * slow = head;
+    node * fast = head;
+    while(k--)
+    {
+        fast = fast->next;
+    }
+    while(fast!=NULL)
+    {
+        fast = fast->next;
+        slow = slow->next;
+    }
+    return slow;
+}
+
+int main() {
+#ifndef ONLINE_JUDGE
+    freopen("errorf.in", "w", stderr);
+#endif
+
+    fastio();
+    node*head;
+    cin>>head;
+    cout<<head;
+    cout<<nline;
+    reverse_ll(head);
+    cout<<head;
+    // node*head2;
+
+    // cin>>head2;
+    // cout<<nline;
+    // cout<<head2;
+}
diff --git a/Day_31_to_40/Day_40/outputf.in b/Day_31_to_40/Day_40/outputf.in
index 3df68c9..065ac31 100644
--- a/Day_31_to_40/Day_40/outputf.in
+++ b/Day_31_to_40/Day_40/outputf.in
@@ -1 +1,3 @@
-0 1 1 12 
\ No newline at end of file
+3, 2, 1, 
+
+1, 2, 3, 
diff --git a/Day_31_to_40/Day_40/reverse_ll_itr.cpp b/Day_31_to_40/Day_40/reverse_ll_itr.cpp
new file mode 100644
index 0000000..e3f4d95
--- /dev/null
+++ b/Day_31_to_40/Day_40/reverse_ll_itr.cpp
@@ -0,0 +1,183 @@
+// https://youtu.be/8ymiMHQPgZY
+// https://github.com/darkprinx/ACM-SOLUTION/blob/master/Spoj%20Solutions/FILRTEST%20-%20File%20Recover%20Testing.cpp
+#include<bits/stdc++.h>
+
+using namespace std;
+
+// void _print(ll t) {cerr << t;}
+// void _print(int t) {cerr << t;}
+// void _print(string t) {cerr << t;}
+// void _print(char t) {cerr << t;}
+// void _print(lld t) {cerr << t;}
+// void _print(double t) {cerr << t;}
+// void _print(ull t) {cerr << t;}
+
+// template <class T, class V> void _print(pair <T, V> p);
+// template <class T> void _print(vector <T> v);
+// template <class T> void _print(set <T> v);
+// template <class T, class V> void _print(map <T, V> v);
+// template <class T> void _print(multiset <T> v);
+// template <class T, class V> void _print(pair <T, V> p) {cerr << "{"; _print(p.ff); cerr << ","; _print(p.ss); cerr << "}";}
+// template <class T> void _print(vector <T> v) {cerr << "[ "; for (T i : v) {_print(i); cerr << " ";} cerr << "]";}
+// template <class T> void _print(set <T> v) {cerr << "[ "; for (T i : v) {_print(i); cerr << " ";} cerr << "]";}
+// template <class T> void _print(multiset <T> v) {cerr << "[ "; for (T i : v) {_print(i); cerr << " ";} cerr << "]";}
+// template <class T, class V> void _print(map <T, V> v) {cerr << "[ "; for (auto i : v) {_print(i); cerr << " ";} cerr << "]";}
+
+#ifndef ONLINE_JUDGE
+#define debug(x) cerr << #x <<" "; _print(x); cerr << endl;
+#else
+#define debug(x)
+#endif
+
+
+#define fastio() ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
+#define MOD 1000000007
+#define MOD1 998244353
+#define INF 1e18
+#define nline "\n"
+#define pb push_back
+#define ppb pop_back
+#define mp make_pair
+#define ff first
+#define ss second
+#define PI 3.141592653589793238462
+#define set_bits __builtin_popcountll
+#define sz(x) ((int)(x).size())
+#define all(x) (x).begin(), (x).end()
+#define my_sizeof(type) ((char *)(&type+1)-(char*)(&type))
+#define print_vec(x) for(auto i:x) cout<<i<<" "; // prints elements of an int vector
+#define check_all_zero(v) all_of(v.begin(), v.end(), [](int i) { return i==0; })
+#define str_to_int(x) stoi(x) // converts a string to an int
+#define eps 1e-9
+#define gcd(x,y) __gcd(x,y)
+#define lcm(a,b) (a*(b/gcd(a,b)))
+#define maxall(v) *max_element(all(v))
+#define minall(v) *min_element(all(v))
+#define max3(a,b,c) max(a,max(b,c))
+#define min3(a,b,c) min(a,min(b,c))
+
+
+typedef long long ll;
+typedef long long int lli;
+typedef unsigned long long ull;
+typedef long double lld;
+typedef vector<int> vi;
+typedef vector<ll> vll;
+typedef pair<int, int> pii;
+typedef pair<ll, ll> pll;
+typedef pair<ll, bool> pib;
+typedef pair<double, double> pdd;
+typedef pair<int, double> pid;
+typedef pair< int, pii> ipii;
+// typedef tree<pair<int, int>, null_type, less<pair<int, int>>, rb_tree_tag, tree_order_statistics_node_update > pbds; // find_by_order, order_of_key
+// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Code Below ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~//
+
+class node{
+public:
+    int data;
+    node*next;
+
+    node(int d){
+        data = d;
+        next = NULL;
+    }
+    ~node(){
+        if(next!=NULL){
+            delete next;
+        }
+    }
+};
+
+void print_ll(node* head){   // at the print time we dont pass by reference
+    while(head!=NULL){
+        cout<<head->data<<", ";
+        head = head->next;
+    }
+    cout<<endl;
+}
+
+void insert_head(node*& head,int d){    //passing by reference
+    if(head == NULL){
+        head = new node(d);             // creating node in dynamic memory
+                                        // which only returns the address of
+                                        // created node
+        return;
+    }
+
+    node *n = new node(d);
+    n->next = head;
+    head= n;
+}
+
+node* take_input(){
+    int d;
+    cin>>d;                     // taking the data
+    node* head = NULL;          // creating the head pointer
+
+    while(d!=-1){              // - + untill user provides -1 as the input we ill keep taking input from user
+        insert_head(head,d);   //   | and insert in the head(we can use other insert operations too)
+        cin>>d;                //   | and take input from the user, which will eventually update d.
+    }                          // - +
+    return head;               // we return head as we are not taking head in the function input.
+}
+
+
+node* take_input_txt(){
+    int d;
+    node* head = NULL;
+
+    cin>>d;
+    while(d!=-1){
+        insert_head(head,d);
+        cin>>d;
+    }
+
+    return head;
+}
+
+
+ostream& operator<<(ostream &os,node* head){
+    print_ll(head);
+    return os;
+}
+
+istream& operator>>(istream &is, node*&head){
+    head = take_input_txt();
+    return is;
+}
+
+
+// reverse Linked List using recursion
+void reverse_ll(node* &head){
+    // think this is a ll: 1->2->3->NULL
+    node* prev = NULL;
+    node* c = head;
+    node* n;
+    while(c !=NULL){
+        n = c->next;  // now n is pointing to 2
+        c->next = prev;
+        prev = c;
+        c =n;
+    }
+    head = prev;
+    return;
+}
+
+int main() {
+#ifndef ONLINE_JUDGE
+    freopen("errorf.in", "w", stderr);
+#endif
+
+    fastio();
+    node*head;
+    cin>>head;
+    cout<<head;
+    cout<<nline;
+    reverse_ll(head);
+    cout<<head;
+    // node*head2;
+
+    // cin>>head2;
+    // cout<<nline;
+    // cout<<head2;
+}
diff --git a/Day_31_to_40/Day_40/reverse_ll_itr.exe b/Day_31_to_40/Day_40/reverse_ll_itr.exe
new file mode 100644
index 0000000..d7db610
Binary files /dev/null and b/Day_31_to_40/Day_40/reverse_ll_itr.exe differ
diff --git a/Day_31_to_40/Day_40/reverse_ll_rec_1.cpp b/Day_31_to_40/Day_40/reverse_ll_rec_1.cpp
new file mode 100644
index 0000000..c8acc36
--- /dev/null
+++ b/Day_31_to_40/Day_40/reverse_ll_rec_1.cpp
@@ -0,0 +1,202 @@
+// https://youtu.be/8ymiMHQPgZY
+// https://github.com/darkprinx/ACM-SOLUTION/blob/master/Spoj%20Solutions/FILRTEST%20-%20File%20Recover%20Testing.cpp
+#include<bits/stdc++.h>
+
+using namespace std;
+
+// void _print(ll t) {cerr << t;}
+// void _print(int t) {cerr << t;}
+// void _print(string t) {cerr << t;}
+// void _print(char t) {cerr << t;}
+// void _print(lld t) {cerr << t;}
+// void _print(double t) {cerr << t;}
+// void _print(ull t) {cerr << t;}
+
+// template <class T, class V> void _print(pair <T, V> p);
+// template <class T> void _print(vector <T> v);
+// template <class T> void _print(set <T> v);
+// template <class T, class V> void _print(map <T, V> v);
+// template <class T> void _print(multiset <T> v);
+// template <class T, class V> void _print(pair <T, V> p) {cerr << "{"; _print(p.ff); cerr << ","; _print(p.ss); cerr << "}";}
+// template <class T> void _print(vector <T> v) {cerr << "[ "; for (T i : v) {_print(i); cerr << " ";} cerr << "]";}
+// template <class T> void _print(set <T> v) {cerr << "[ "; for (T i : v) {_print(i); cerr << " ";} cerr << "]";}
+// template <class T> void _print(multiset <T> v) {cerr << "[ "; for (T i : v) {_print(i); cerr << " ";} cerr << "]";}
+// template <class T, class V> void _print(map <T, V> v) {cerr << "[ "; for (auto i : v) {_print(i); cerr << " ";} cerr << "]";}
+
+#ifndef ONLINE_JUDGE
+#define debug(x) cerr << #x <<" "; _print(x); cerr << endl;
+#else
+#define debug(x)
+#endif
+
+
+#define fastio() ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
+#define MOD 1000000007
+#define MOD1 998244353
+#define INF 1e18
+#define nline "\n"
+#define pb push_back
+#define ppb pop_back
+#define mp make_pair
+#define ff first
+#define ss second
+#define PI 3.141592653589793238462
+#define set_bits __builtin_popcountll
+#define sz(x) ((int)(x).size())
+#define all(x) (x).begin(), (x).end()
+#define my_sizeof(type) ((char *)(&type+1)-(char*)(&type))
+#define print_vec(x) for(auto i:x) cout<<i<<" "; // prints elements of an int vector
+#define check_all_zero(v) all_of(v.begin(), v.end(), [](int i) { return i==0; })
+#define str_to_int(x) stoi(x) // converts a string to an int
+#define eps 1e-9
+#define gcd(x,y) __gcd(x,y)
+#define lcm(a,b) (a*(b/gcd(a,b)))
+#define maxall(v) *max_element(all(v))
+#define minall(v) *min_element(all(v))
+#define max3(a,b,c) max(a,max(b,c))
+#define min3(a,b,c) min(a,min(b,c))
+
+
+typedef long long ll;
+typedef long long int lli;
+typedef unsigned long long ull;
+typedef long double lld;
+typedef vector<int> vi;
+typedef vector<ll> vll;
+typedef pair<int, int> pii;
+typedef pair<ll, ll> pll;
+typedef pair<ll, bool> pib;
+typedef pair<double, double> pdd;
+typedef pair<int, double> pid;
+typedef pair< int, pii> ipii;
+// typedef tree<pair<int, int>, null_type, less<pair<int, int>>, rb_tree_tag, tree_order_statistics_node_update > pbds; // find_by_order, order_of_key
+// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Code Below ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~//
+
+class node{
+public:
+    int data;
+    node*next;
+
+    node(int d){
+        data = d;
+        next = NULL;
+    }
+    ~node(){
+        if(next!=NULL){
+            delete next;
+        }
+    }
+};
+
+void print_ll(node* head){   // at the print time we dont pass by reference
+    while(head!=NULL){
+        cout<<head->data<<", ";
+        head = head->next;
+    }
+    cout<<endl;
+}
+
+void insert_head(node*& head,int d){    //passing by reference
+    if(head == NULL){
+        head = new node(d);             // creating node in dynamic memory
+                                        // which only returns the address of
+                                        // created node
+        return;
+    }
+
+    node *n = new node(d);
+    n->next = head;
+    head= n;
+}
+
+node* take_input(){
+    int d;
+    cin>>d;                     // taking the data
+    node* head = NULL;          // creating the head pointer
+
+    while(d!=-1){              // - + untill user provides -1 as the input we ill keep taking input from user
+        insert_head(head,d);   //   | and insert in the head(we can use other insert operations too)
+        cin>>d;                //   | and take input from the user, which will eventually update d.
+    }                          // - +
+    return head;               // we return head as we are not taking head in the function input.
+}
+
+
+node* take_input_txt(){
+    int d;
+    node* head = NULL;
+
+    cin>>d;
+    while(d!=-1){
+        insert_head(head,d);
+        cin>>d;
+    }
+
+    return head;
+}
+
+
+ostream& operator<<(ostream &os,node* head){
+    print_ll(head);
+    return os;
+}
+
+istream& operator>>(istream &is, node*&head){
+    head = take_input_txt();
+    return is;
+}
+
+
+// reverse Linked List using recursion
+void reverse_ll(node* &head){
+    // think this is a ll: 1->2->3->NULL
+    node* prev = NULL;
+    node* c = head;
+    node* n;
+    while(c !=NULL){
+        n = c->next;  // now n is pointing to 2
+        c->next = prev;
+        prev = c;
+        c =n;
+    }
+    head = prev;
+    return;
+}
+
+node* reverse_rec(node* &head){
+    if(head=NULL or head->next == NULL){
+        return head;
+    }
+
+    // do recurcive case
+    node* shead = reverse_rec(head->next);
+    node* temp = shead;
+    while(temp->next!=NULL){
+        temp = temp->next;
+    }
+    head->next = NULL;
+    temp->next = head;
+
+    return shead;
+}
+
+
+
+int main() {
+#ifndef ONLINE_JUDGE
+    freopen("errorf.in", "w", stderr);
+#endif
+
+    fastio();
+    node*head;
+    cin>>head;
+    cout<<head;
+    cout<<nline;
+    reverse_ll(head);
+    cout<<head;
+    // node*head2;
+
+    // cin>>head2;
+    // cout<<nline;
+    // cout<<head2;
+}
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