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

题目	Balanced Binary Tree
通过率	32.1%
难度	Easy

题目是判断是否是平衡二叉树

平衡二叉树定义（AVL）：它或者是一颗空树，或者具有以下性质的二叉树：它的左子树和右子树的深度之差的绝对值不超过1，且它的左子树和右子树都是一颗平衡二叉树。

关键点：深度优先遍历、递归；

我们的判断过程从定义入手即可：

 1. 若为空树，则为true；

 2. 若不为空树，调用getHeight()方法，若为二叉树此方法返回的是树的高度，否则返回-1；

 3. 在getHeight()方法中，利用递归的思想处理左右子树；

java代码：

/**
 * 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;
    }
}