606. Construct String from Binary Tree

https://leetcode.com/problems/construct-string-from-binary-tree/

二叉树表示成由()和数字组成的字符串，数字代表二叉树结点的值，数字旁边分别是左右子树，每个子树是括号括起来的递归定义的树，如果只有右子树则需要引入一个空()。根据题目规定做pre-order 遍历，返回的str外加()。

/*
 * @lc app=leetcode id=606 lang=cpp
 *
 * [606] Construct String from Binary Tree
 */

// @lc code=start
/**
 * 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:
    string tree2str(TreeNode* t) {
        if (t == nullptr) return "";
        string res;
        res += to_string(t->val);
        if (t->left != nullptr) res += "(" + tree2str(t->left) + ")";
        if (t->right != nullptr) {
            if (t->left == nullptr) res += "()";
            res += "(" + tree2str(t->right) + ")";
        }
        return res;
    }
};
// @lc code=end