leetcode-606-easy

Construct String from Binary Tree

Given the root of a binary tree, construct a string consisting of parenthesis and integers from a binary tree with the preorder traversal 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: Originally, 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

思路一：先序遍历，递归拼接字符串

    public String tree2str(TreeNode root) {
        if (root == null) {
            return "";
        }

        String ans = root.val + "";
        if (root.left != null) {
            if (root.right == null) {
                ans += "(" + tree2str(root.left) + ")";
            } else {
                ans += "(" + tree2str(root.left) + ")" + "(" + tree2str(root.right) + ")";
            }
        } else {
            if (root.right != null) {
                ans += "()(" + tree2str(root.right) + ")";
            }
        }

        return ans;
    }