Mini Parser Solutions in C++

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




/**
 * // 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');
    }
};