536. Construct Binary Tree from String

/**
 * 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) {
        int pos = 0;
        return build(s, pos);
    }
private:
    TreeNode* build(string s, int& p) {
        if (p == s.size()) return nullptr;  //pay attention to the base condition! not (!isdigit(s[p])
        int val = 0, sign = 1;  //pay attention to negative num!!!
        if (s[p] == '-') p++, sign = -1;
        while (isdigit(s[p])) val = val * 10 + s[p++] - '0';
        TreeNode* root = new TreeNode(val*sign);
        if (s[p] == '(')
            root->left = build(s, ++p);
        if (s[p] == '(')
            root->right = build(s, ++p);
        ++p;
        return root;
    }
};