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.

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思路：题意是要判断一棵树是否是二叉平衡树，即任意一节点的左右子树的高度差不得超过1，空树算是一种特殊的平衡二叉树。明显还是需要DFS来后序遍历，只要有节点的左右子树高度相差1以上，那么这棵树就一定是非平衡的。

代码如下：

/** * Definition for a binary tree node. * 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(getDeep(root)==-1){            return false;        }                return true;    }        public int getDeep(TreeNode root){        if(root==null) return 0;                int left = getDeep(root.left);        int right = getDeep(root.right);                if(left==-1 || right==-1){            return -1;        }                if(Math.abs(left-right)>1){            return -1;        }                return Math.max(left,right)+1;    }}