110 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.

Solution

/**
 * 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:
    int depth(TreeNode* root) {
        if ( root == NULL ) return 0;
        int left = depth(root->left), right = depth(root->right);
        if ( left == -1 or right == -1 ) return -1;
        if ( abs(left-right) >= 2 ) return -1;
        return max(left, right) + 1;
    }
    bool isBalanced(TreeNode* root) {
        if ( root == NULL ) return true;
        int left = depth(root->left), right = depth(root->right);
        if ( left == -1 or right == -1 ) return false;
        if ( abs(left-right) >= 2 ) return false;
        return true;
    }
};

Notes
Note the trick to return -1 when the tree is not balanced.