Balanced Binary Tree Solutions in C++

// Source : https://oj.leetcode.com/problems/balanced-binary-tree/
// Author : Hao Chen
// Date   : 2014-06-28



/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isBalanced(TreeNode *root) {
        int height=0;
        return isBalancedUtil(root, height);
    }

    bool isBalancedUtil(TreeNode* root, int& height){
       if(root==NULL){
          height=0;
          return true;
       }
       int lh=0, rh=0;
       bool isLeft = isBalancedUtil(root->left, lh);
       bool isRight = isBalancedUtil(root->right, rh);
       height = (lh > rh ? lh : rh) + 1;
       return (abs(lh-rh)<=1 && isLeft && isRight);
    }

};

//Notes: 
// I think the above solution should be more efficent than the below, 
//  but for leetcode, the below solution needs 60ms, the above needs 88ms
class Solution {
public:
    bool isBalanced(TreeNode *root) {
        if (root==NULL) return true;
        
        int left = treeDepth(root->left); 
        int right = treeDepth(root->right);
        
        if (left-right>1 || left-right < -1) {
            return false;
        }
        return isBalanced(root->left) && isBalanced(root->right);
    }
    
    int treeDepth(TreeNode *root) {
        if (root==NULL){
            return 0;
        }

        int left=1, right=1;
        
        left += treeDepth(root->left);
        right += treeDepth(root->right);
        
        return left>right?left:right;
    }

};

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */

#include <complex> 

struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
}; 
 
class Solution {
public:
    bool isBalanced(TreeNode *root) {
        height(root);
        return bBalanced;    
    }
private:  
    int height(TreeNode *node)
    {
        if (node == NULL || !bBalanced) return 0;
        else
        {
            int leftHeight = height(node->left);
            int rightHeight = height(node->right);
            if (std::abs(leftHeight - rightHeight) > 1) bBalanced = false;
            return std::max(leftHeight, rightHeight) + 1;
        }
    }
    
    bool bBalanced = true;
};