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.

 

Analyse: Recursively judge

             1. height difference of subtrees of the current root larger than 1

　　　　  2. whether the current root has balanced subtrees.

Runtime: 20ms.

1 /** 2  * Definition for a binary tree node. 3  * struct TreeNode { 4  *     int val; 5  *     TreeNode *left; 6  *     TreeNode *right; 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} 8  * }; 9  */10 class Solution {11 public:12     bool isBalanced(TreeNode* root) {13         if(!root) return true;14         if(!root->left && !root->right) return true;15         16         if(abs(depth(root->left) - depth(root->right)) > 1) return false;17         return isBalanced(root->left) && isBalanced(root->right);18     }19     int depth(TreeNode* root){20         if(!root) return 0;21         return max(depth(root->left), depth(root->right)) + 1;22     }23 };