leetcode[156]Binary Tree Upside Down

Given a binary tree where all the right nodes are either leaf nodes with a sibling (a left node that shares the same parent node) or empty, flip it upside down and turn it into a tree where the original right nodes turned into left leaf nodes. Return the new root.

For example:
Given a binary tree {1,2,3,4,5},
1
/ \
2 3
/ \
4 5

return the root of the binary tree [4,5,2,#,#,3,1].
4
/ \
5 2
/ \
3 1

TreeNode *upsideDownBinaryTree(TreeNode *root)
{
if(root == NULL)
return root;

stack<TreeNode*> s;    //left child list
s.push(root);
TreeNode* cur = root;
while(cur->left)
{
s.push(cur->left);
cur = cur->left;
}
TreeNode* newroot = s.top();
cur = newroot;
s.pop();
while(!s.empty())
{
TreeNode* oldfather = s.top();
s.pop();
cur->left = oldfather->right;
cur->right = oldfather;
oldfather->left=NULL;
oldfather->right=NULL;
cur = curr->left;
}
return newroot;
}