MiniParser.cpp

// Source : https://leetcode.com/problems/mini-parser/
// Author : Hao Chen
// Date   : 2016-08-24

/*************************************************************************************** 
 *
 * Given a nested list of integers represented as a string, implement a parser to 
 * deserialize it.
 * 
 * Each element is either an integer, or a list -- whose elements may also be integers 
 * or other lists.
 * 
 * Note:
 * You may assume that the string is well-formed:
 * 
 * String is non-empty.
 * String does not contain white spaces.
 * String contains only digits 0-9, [, - ,, ].
 * 
 * Example 1:
 * 
 * Given s = "324",
 * 
 * You should return a NestedInteger object which contains a single integer 324.
 * 
 * Example 2:
 * 
 * Given s = "[123,[456,[789]]]",
 * 
 * Return a NestedInteger object containing a nested list with 2 elements:
 * 
 * 1. An integer containing value 123.
 * 2. A nested list containing two elements:
 *     i.  An integer containing value 456.
 *     ii. A nested list with one element:
 *          a. An integer containing value 789.
 ***************************************************************************************/


/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * class NestedInteger {
 *   public:
 *     // Constructor initializes an empty nested list.
 *     NestedInteger();
 *
 *     // Constructor initializes a single integer.
 *     NestedInteger(int value);
 *
 *     // Return true if this NestedInteger holds a single integer, rather than a nested list.
 *     bool isInteger() const;
 *
 *     // Return the single integer that this NestedInteger holds, if it holds a single integer
 *     // The result is undefined if this NestedInteger holds a nested list
 *     int getInteger() const;
 *
 *     // Set this NestedInteger to hold a single integer.
 *     void setInteger(int value);
 *
 *     // Set this NestedInteger to hold a nested list and adds a nested integer to it.
 *     void add(const NestedInteger &ni);
 *
 *     // Return the nested list that this NestedInteger holds, if it holds a nested list
 *     // The result is undefined if this NestedInteger holds a single integer
 *     const vector<NestedInteger> &getList() const;
 * };
 */
class Solution {
public:
    NestedInteger deserialize(string s) {
        if (s.size()==0) return NestedInteger();
        int pos = 0;
        if (s[pos]!='[') return atoni(s, pos);
        
        return helper(s, ++pos);
    }
private:
    NestedInteger helper(string& s, int& pos) {
       
        NestedInteger ni;
        
        while ( s[pos] != ']' && pos < s.size() ) {

            if (s[pos]=='-' || isnum(s[pos])){
                ni.add(atoni(s, pos));
            }else if (s[pos] == '[') {
                pos++;
                ni.add(helper(s, pos));
            }else {
                pos++;
            }
        }
        pos++;
        return ni;
    }
    NestedInteger atoni(string& s, int& pos) {
        int sign = 1;
        int num = 0;
        if (s[pos]=='-') {
            sign = -1;
            pos++;
        }
        for (; pos < s.size(); pos++) {
            if (isnum(s[pos])) {
                num = num * 10 + s[pos] - '0';
            }else{
                break;
            }
        }
        return NestedInteger(sign * num);
    }
    bool isnum(char& c) {
        return (c >='0' && c <='9');
    }
};