LeetCode.110.Balanced Binary Tree 是否平衡二叉树

题目描述

110 Balanced Binary Tree
https://leetcode-cn.com/problems/balanced-binary-tree/

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:
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.

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解题过程

类似求二叉树的高度 LeetCode.104.MaximumDepthOfBinaryTree
只不过在递归的过程中，就可以判断当前子树是否平衡，如果不平衡直接返回负值快速失败结束计算。

private static class SolutionV2020 {
    public boolean isBalanced(TreeNode root) {
        return height(root) >= 0;
    }

    // 计算树的高度，同时判断是否平衡二叉树。若是平衡树返回树高度，若非平衡树返回负值
    private int height(TreeNode root) {
        if (null == root) {
            return 0;
        }
        int leftHeight = height(root.left);
        int rightHeight = height(root.right);
        // 左右子树非平衡树，或者左右子树高度差大于1，则当前树不是平衡树
        if (leftHeight < 0 || rightHeight < 0 || Math.abs(leftHeight - rightHeight) > 1) {
            return -1;
        }
        return Math.max(leftHeight, rightHeight) + 1;
    }
}

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GitHub代码

algorithms/leetcode/leetcode/_110_BalancedBinaryTree.java
https://github.com/masikkk/algorithms/blob/master/leetcode/leetcode/_110_BalancedBinaryTree.java

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