// Recursionclass Solution {public:    TreeNode *upsideDownBinaryTree(TreeNode *root) {        if (!root || !root->left) return root;        TreeNode *l = root->left, *r = root->right;        TreeNode *res = upsideDownBinaryTree(l);        l->left = r;        l->right = root;        root->left = NULL;        root->right = NULL;        return res;    }};

// Iterativeclass Solution {public:    TreeNode *upsideDownBinaryTree(TreeNode *root) {        TreeNode *cur = root, *pre = NULL, *next = NULL, *tmp = NULL;        while (cur) {            next = cur->left;            cur->left = tmp;            tmp = cur->right;            cur->right = pre;            pre = cur;            cur = next;        }        return pre;    }};