判断二叉树是否平衡

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 everynode never differ by more than 1.

刚开始的时候老往AVL树上靠，后来才知道只需要两个子树的高度 never differ by more than 1，这个就好理解多了。代码如下：

    bool balance(TreeNode* root, int& depth)
    {
        if (root == NULL) 
        {
            depth = 0;
            return true;
        }
        int left_h = 0;
        int right_h = 0;
        if (!balance(root->right, right_h))
        {
            return false;
        }
        if (!balance(root->left, left_h))
        {
            return false;
        }
        if (right_h > left_h)
        {
            depth = right_h + 1;
            return (right_h - left_h) <= 1;
        }
        else
        {
            depth = left_h + 1;
            return (left_h - right_h) <= 1;
        }
    }