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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/**
 * 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) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        int h = 0;
        return isBalanced(root, h);
    }
    
private:
    bool isBalanced(TreeNode *root, int &h) {
        if (root == NULL) {
            h = 0;
            return true;
        }
        
        if (root->left == NULL && root->right == NULL) {
            h = 1;
            return true;
        }
        
        int l = 0, r = 0;
        bool ret = true;
        if (!isBalanced(root->left, l) || !isBalanced(root->right, r)) ret = false;
        h = max(l, r) + 1;
        return ret && abs(l - r) <= 1;
    }
};