印象中平衡樹也是大學修資料結構時必考的問題呢．

問題

https://leetcode.com/problems/balanced-binary-tree/description/

Given a binary tree, determine if it is height-balanced.

For this problem, a height-balanced binary tree is defined as:

a binary tree in which the left and right subtrees of every node differ in height by no more than 1.

Example 1:
Input: root = [3,9,20,null,null,15,7]
Output: true

Example 2:
Input: root = [1,2,2,3,3,null,null,4,4]
Output: false

Example 3:
Input: root = []
Output: true

Constraints:

• The number of nodes in the tree is in the range [0, 5000].
• -104 <= Node.val <= 104

我的答案

Python

# 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 isBalanced(self, root: Optional[TreeNode]) -> bool:
        if not root:
            return True
        return self.check(root) != -1

    def check(self, root):
        # The height of leaf node is 0
        if root is None:
            return 0
        # Get the height of left and right child nodes
        left  = self.check(root.left)
        right = self.check(root.right)

        # Return -1 if child nodes' height is not balanced
        if left == -1 or right == -1 or abs(left - right) > 1:
            return -1
        
        return 1 + max(left, right)