Leetcode: Balanced Binary Tree

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution
{
private:
    bool checkBalanced(TreeNode *node, int &depth)
    {
        if (node == NULL)
        {
            depth = 0;
            return true;
        }

        int leftDepth, rightDepth;
        bool leftBalanced = checkBalanced(node->left, leftDepth);
        bool rightBalanced = checkBalanced(node->right, rightDepth);

        depth = max(leftDepth, rightDepth) + 1;//求以node为根节点的深度
        return leftBalanced && rightBalanced && abs(leftDepth - rightDepth) <= 1;
    }
public:
    bool isBalanced(TreeNode *root)
    {
        int depth;
        return checkBalanced(root, depth);
    }
};