Flatten Binary Tree to Linked List

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
先对二叉树进行前序排列，然后放进一个只有右支树的二叉树中

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root: a TreeNode, the root of the binary tree
     * @return: nothing
     */
    public void flatten(TreeNode root) {
        // write your code here
        if (root == null)   return;  
        TreeNode cur =  root , pre = null;
        while(cur != null){  //当前节点不为空
            if ( cur.left != null){  //当前节点有左儿子
                pre = cur.left;   
                while ( pre.right !=null ) pre = pre.right;  //当前节点的右儿子指向左儿子，左儿子的最右儿子指向当前节点的右儿子
                pre.right = cur.right;
                cur.right = cur.left;
                cur.left  = null;
            }
            cur=cur.right;
        }
    }
}