110. Balanced Binary Tree

难度: Easy

题目描述

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

 

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

解题思路

代码实现

解决方案

java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public boolean isBalanced(TreeNode root) {
        if (root == null) {
            return true;
        }
        TreeNode left = root.left;
        TreeNode right = root.right;
        int leftHeight = getHeight(left);
        int rightHeight = getHeight(right);
        if (Math.abs(leftHeight - rightHeight) > 1) {
            return false;
        }
        return isBalanced(left) && isBalanced(right);
    }

    // 计算树的高度
    private int getHeight(TreeNode node) {
        if (node == null) {
            return 0;
        }
        return Math.max(getHeight(node.left), getHeight(node.right)) + 1;
    }

}