struct TreeNode
{
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

class Solution
{
public:
    TreeNode *upsideDownBinaryTree(TreeNode *root)
    {
        if (!root || !root->left)
            return root;
        TreeNode *l = root->left; //左右子节点存取来
        TreeNode *r = root->right;
        root->left = NULL; //上下断开
        root->right = NULL;
        TreeNode *p = upsideDownBinaryTree(l); //根节点
        l->left = r;                           //跟上面连接起来
        l->right = root;
        return p;
    }
};