Leetcode606. Construct String from Binary Tree

题目：

You need to construct a string consists of parenthesis and integers from a binary tree with the preorder traversing way.

The null node needs to be represented by empty parenthesis pair "()". And you need to omit all the empty parenthesis pairs that don't affect the one-to-one mapping relationship between the string and the original binary tree.

Example 1:

Input: Binary tree: [1,2,3,4]       1     /   \    2     3   /      4     Output: "1(2(4))(3)"
Explanation: Originallay it needs to be "1(2(4)())(3()())", 
but you need to omit all the unnecessary empty parenthesis pairs. 
And it will be "1(2(4))(3)".

Example 2:

Input: Binary tree: [1,2,3,null,4]       1     /   \    2     3     \        4 Output: "1(2()(4))(3)"
Explanation: Almost the same as the first example, 
except we can't omit the first parenthesis pair to break the one-to-one mapping relationship between the input and the output.

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题目分析：关于树的算法题，首先第一步考虑递归算法。这道题也不例外，也是用递归去实现。

代码：

class Solution {public:    string tree2str(TreeNode* t) {        if (t == NULL) return "";        string s = to_string(t->val);        if (t->left) s += '(' + tree2str(t->left) + ')';        else if (t->right) s += "()";        if (t->right) s += '(' + tree2str(t->right) + ')';        return s;    }};