压扁二叉树

原题地址
Given a binary tree, flatten it to a linked list in-place.

For example, given the following tree:

    1
   / 
  2   5
 /    
3   4   6

The flattened tree should look like:

1
 
  2
   
    3
     
      4
       
        5
         
          6
          ```

一种非常巧妙的方法：

TreeNode pre = null;
public void traverse(TreeNode node){
if(node == null)
return;
traverse(node.right);
traverse(node.left);

node.left = null;
node.right = pre;

pre = node; } ```