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.

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.

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isBalanced(TreeNode* root) {
        if(!root) return true;
        if(!root->left && !root->right) return true;
        
        if(abs(depth(root->left) - depth(root->right)) > 1) return false;
        return isBalanced(root->left) && isBalanced(root->right);
    }
    int depth(TreeNode* root){
        if(!root) return 0;
        return max(depth(root->left), depth(root->right)) + 1;
    }
};