Construct Binary Tree from String.cpp

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* str2tree(string s) {
        s = "(" + s + ")";
        int len = s.size();
        return s2T(s, 0, len - 1);
    }

private:
    TreeNode* s2T(string s, int lo, int hi)
    {
        if(lo + 1 > hi - 1)return nullptr;
        TreeNode* node = nullptr;
        int num = 0, i = lo + 1;
        bool isNeg = s[i] == '-'? true: false;
        if(isNeg)++i;
        
        while(i < hi && isdigit(s[i]))
            num = num * 10 + (s[i++] - '0');

        node = new TreeNode(isNeg? -num: num);
        if(i == hi)return node;//leaf node
        int count = 0, j = i;
        while(j <= hi)
        {
            if(s[j] == '(')++count;
            else if(s[j] == ')') --count;
            if(!count)break;
            ++j;
        }
        node->left = s2T(s, i, j);
        node->right = s2T(s, j + 1, hi - 1);
        return node;
    }
};