z.cpp

// A C++ program that implements Z algorithm for pattern searching 
#include<iostream> 
using namespace std; 

void getZarr(string str, int Z[]); 

// prints all occurrences of pattern in text using Z algo 
void search(string text, string pattern) 
{ 
	// Create concatenated string "P$T" 
	string concat = pattern + "$" + text; 
	int l = concat.length(); 

	// Construct Z array 
	int Z[l]; 
	getZarr(concat, Z); 

	// now looping through Z array for matching condition 
	for (int i = 0; i < l; ++i) 
	{ 
		// if Z[i] (matched region) is equal to pattern 
		// length we got the pattern 
		if (Z[i] == pattern.length()) 
			cout << "Pattern found at index "
				<< i - pattern.length() -1 << endl; 
	} 
} 

// Fills Z array for given string str[] 
void getZarr(string str, int Z[]) 
{ 
	int n = str.length(); 
	int L, R, k; 

	// [L,R] make a window which matches with prefix of s 
	L = R = 0; 
	for (int i = 1; i < n; ++i) 
	{ 
		// if i>R nothing matches so we will calculate. 
		// Z[i] using naive way. 
		if (i > R) 
		{ 
			L = R = i; 

			// R-L = 0 in starting, so it will start 
			// checking from 0'th index. For example, 
			// for "ababab" and i = 1, the value of R 
			// remains 0 and Z[i] becomes 0. For string 
			// "aaaaaa" and i = 1, Z[i] and R become 5 
			while (R<n && str[R-L] == str[R]) 
				R++; 
			Z[i] = R-L; 
			R--; 
		} 
		else
		{ 
			// k = i-L so k corresponds to number which 
			// matches in [L,R] interval. 
			k = i-L; 

			// if Z[k] is less than remaining interval 
			// then Z[i] will be equal to Z[k]. 
			// For example, str = "ababab", i = 3, R = 5 
			// and L = 2 
			if (Z[k] < R-i+1) 
				Z[i] = Z[k]; 

			// For example str = "aaaaaa" and i = 2, R is 5, 
			// L is 0 
			else
			{ 
				// else start from R and check manually 
				L = i; 
				while (R<n && str[R-L] == str[R]) 
					R++; 
				Z[i] = R-L; 
				R--; 
			} 
		} 
	} 
} 

// Driver program 
int main() 
{ 
	string text = "GEEKS FOR GEEKS"; 
	string pattern = "GEEK"; 
	search(text, pattern); 
	return 0; 
}