606. Construct String from Binary Tree

Easy

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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

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def tree2str(self, root: Optional[TreeNode]) -> str:
        if not root:
            return ""
        left = ""
        right = ""
        if root.left or root.right:
            left = "(" + self.tree2str(root.left) + ")"
        if root.right:
            right = "(" + self.tree2str(root.right) + ")"
        return f"{root.val}{left}{right}"