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;
   }