110-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.

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Example 1:

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

   3
   / \
  9  20
    /  \
   15   7

Return true.

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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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問題描述

給定一個二叉樹， 判斷它的高度是否平衡

高度平衡的二叉樹定義如下:

每個節點的左右子樹的高度的差值的絕對值不會大於1

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問題分析

在後序遍歷的過程中求高度並進行判斷

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解法

public class Solution {
    public boolean isBalanced(TreeNode root) {        
        return helper(root) != -1;
    }

    private int helper(TreeNode root) {
        if (root == null) return 0;

        int left = helper(root.left);
        int right = helper(root.right);

        if (left == -1 || right == -1 || Math.abs(left - right) > 1) return -1;

        return Math.max(left, right) + 1;
    }
}