110. Balanced Binary Tree

Not much to say, a classic DFS solution on tree.

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 == NULL) return true;  
14:      if (abs(depth(root->left) - depth(root->right)) > 1) return false;  
15:      return isBalanced(root->left) && isBalanced(root->right);  
16:    }  
17:    int depth(TreeNode *root) {  
18:      if (root == NULL) return 0;  
19:      return 1 + max(depth(root->left), depth(root->right));  
20:    }  
21:  };