LeetCode 110. Balanced Binary Tree 平衡二叉树

题目

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 depth of the two subtrees of every node never differ by more than 1.

Example 1:

Given the following tree [3,9,20,null,null,15,7]:

    3
   / \
  9  20
    /  \
   15   7

Return true.

Example 2:

Given the following tree [1,2,2,3,3,null,null,4,4]:

       1
      / \
     2   2
    / \
   3   3
  / \
 4   4

Return false.

讲解

写一个maxDepth()的辅助函数,在计算二叉树的最大深度同时检测二叉树是否平衡。如果二叉树不平衡,maxDepth()返回null。

视频教学

Java参考代码

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isBalanced(TreeNode root) {
        return maxDepth(root) != null;        
    }
    
    private Integer maxDepth(TreeNode root) {
        if (root == null) {
            return 0;
        }
        
        Integer leftDepth = maxDepth(root.left);
        Integer rightDepth = maxDepth(root.right);
        
        if (leftDepth == null || rightDepth == null) {
            return null;
        }
        
        if (Math.abs(leftDepth - rightDepth) > 1) {
            return null;
        }
        
        return Math.max(leftDepth, rightDepth) + 1;
    }
}

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