leetcode----------Balanced Binary Tree

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

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.node

題目 Balanced Binary Tree
經過率 32.1%
難度 Easy

題目是判斷是不是平衡二叉樹this

平衡二叉樹定義(AVL):它或者是一顆空樹,或者具備如下性質的二叉樹:它的左子樹和右子樹的深度之差的絕對值不超過1,且它的左子樹和右子樹都是一顆平衡二叉樹。spa

關鍵點:深度優先遍歷、遞歸;code

咱們的判斷過程從定義入手便可:blog

 1. 若爲空樹,則爲true;遞歸

 2. 若不爲空樹,調用getHeight()方法,若爲二叉樹此方法返回的是樹的高度,不然返回-1;get

 3. 在getHeight()方法中,利用遞歸的思想處理左右子樹;it

 

java代碼:io

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isBalanced(TreeNode root) {
        if(root==null) return true;
        if(getHeight(root) == -1) return false;
        return true;
    }
    
    public int getHeight(TreeNode root){
        if(root == null) return 0;
        
        int left = getHeight(root.left);
        int right = getHeight(root.right);
        if(left==-1 || right==-1) return -1;
        if(Math.abs(left-right)>1) return -1;
        return Math.max(left,right)+1;
    }
}
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