LeetCode: 110. Balanced Binary Tree

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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


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


求左右子树各自的最大深度,然后进行比较,如果它们的深度差小于等于1的话, 则当前层是平衡的,继续比较下一层。

这个方法的 Python 代码类似下面这样:

# 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):
        if root is None:
            return True
        left_height = self.max_height(root.left)
        right_height = self.max_height(root.right)

        return abs(left_height - right_height) <= 1 and self.isBalanced(root.left) and self.isBalanced(root.right)

    def max_height(self, root):
        if root is None:
            return 0
        return 1 + max(self.max_height(root.left), self.max_height(root.right))

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