Solution.cpp

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    TreeNode* str2tree(string s) {
        return dfs(s);
    }

    TreeNode* dfs(string s) {
        if (s == "") return nullptr;
        int p = s.find("(");
        if (p == s.npos) return new TreeNode(stoi(s));
        TreeNode* root = new TreeNode(stoi(s.substr(0, p)));
        int start = p;
        int cnt = 0;
        for (int i = p; i < s.size(); ++i) {
            if (s[i] == '(')
                ++cnt;
            else if (s[i] == ')')
                --cnt;
            if (cnt == 0) {
                if (start == p) {
                    root->left = dfs(s.substr(start + 1, i - start - 1));
                    start = i + 1;
                } else
                    root->right = dfs(s.substr(start + 1, i - start - 1));
            }
        }
        return root;
    }
};