Flatten Binary Tree to Linked List (LeetCode Tree)

Question: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

Idea: Recursion. Brute force and straightforward. Flatten the left tree, flatten the right tree, then connect the left tree with the right tree and set root.left to null.

Time: O(nlgn) (for each recursion, we need to traverse half of the tree) Space: O(n)

Code:

 public class Solution {  
   public void flatten(TreeNode root) {  
     if(root==null)  
       return;  
     flatten(root.left);  
     flatten(root.right);  
     TreeNode cur=root;  
     if(cur.left!=null)  
     {  
       cur=cur.left;  
       while(cur.right!=null)  
       {  
         cur=cur.right;  
       }  
       cur.right=root.right;  
       root.right=root.left;  
       root.left=null;  
     }  
   }  
 }