leetcode[156] binary tree upside down

TreeNode *ans;
TreeNode *helper156(TreeNode *root)
{
if (!root -> left && !root -> right)
{
ans = root; return root;
}
TreeNode *parent = helper156(root -> left);
parent -> left = root -> right;
parent -> right = root;
root -> left = NULL;
root -> right = NULL;
return parent -> right;
}
TreeNode *UpsideDown(TreeNode *root)
{
if (root)
helper156(root);
return ans;
}

TreeNode *UpsideDown2(TreeNode *root)
{
if (!root -> left && !root -> right) return root;
TreeNode *parent = NULL, *parentRight = NULL, *p = root;
while(p)
{
TreeNode *left = p -> left;
p -> left = parentRight;
parentRight = p -> right;
p -> right = parent;
parent = p;
p = left;
}
return parent;
}