0606. Construct String from Binary Tree

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

Description

Given the root of a binary tree, construct a string consists of parenthesis and integers from a binary tree with the preorder traversing way, and return it.

Omit all the empty parenthesis pairs that do not affect the one-to-one mapping relationship between the string and the original binary tree.

Example 1:

**Input:** root = [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:** root = [1,2,3,null,4]
**Output:** "1(2()(4))(3)"
Explanation: Almost the same as the first example, except we cannot omit the first parenthesis pair to break the one-to-one mapping relationship between the input and the output.

Constraints:

• The number of nodes in the tree is in the range [1, 104].

• -1000 <= Node.val <= 1000

ac

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public String tree2str(TreeNode t) {
        // exit
        if (t == null) return "";

        String left = tree2str(t.left);
        String right = tree2str(t.right);
        String res = "" + t.val;
        if (right.length() != 0 || left.length() != 0) res += "(" + left + ")";
        if (right.length() != 0) res += "(" + right + ")";

        return res;
    }
}