You need to construct a binary tree from a string consisting of parenthesis and integers. 

The whole input represents a binary tree. It contains an integer followed by zero, one or two pairs of parenthesis. The integer represents the root's value and a pair of parenthesis contains a child binary tree with the same structure. 

You always start to construct the left child node of the parent first if it exists.

Example:

Input: "4(2(3)(1))(6(5))"Output: return the tree root node representing the following tree:       4     /   \    2     6   / \   /   3   1 5

/** * 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) {        if(s.size()==0) return NULL;        int pos = s.find_first_of('(');        if(pos==std::string::npos)        {            TreeNode*root = new TreeNode(stoll(s));            return root;        }        int val = stoll(s.substr(0,pos));        stack
     
      stk;        stk.push(s[pos]);        pos++;//skip '('        int left = pos;        while(!stk.empty()&&pos++ < s.size())        {            if(pos
      
       left = str2tree(leftpart);        if(pos+2
       
        0) rightpart.pop_back();            root->right = str2tree(rightpart);        }        return root;    }};