536. Construct Binary Tree from String

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