Flatten binary tree to linked list

LeetCode Flatten Binary Tree to Linked List, Oct 14 '12
难度3，出现频率3
Given a binary tree, flatten it to a linked list in-place.
For example,
Given

         1
        / \
       2   5
      / \   \
     3   4   6

The flattened tree should look like:

   1
    \
     2
      \
       3
        \
         4
          \
           5
            \
             6

思路： 用递归挺简单的--第一个一次通过bug free的代码，oh yeah! 

void flatten(TreeNode *root) {
      if(root == NULL) return;  

      flatten(root->left);

      flatten(root->right);

      TreeNode *temp = root->right; 

      root->right = root->left;

      root->left = NULL;
      TreeNode *cur = root;

      while(cur->right != NULL){

        cur = cur->right;

      } 

      cur->right = temp;
    }