[leetcode]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

算法思想：很明显的先序遍历。

思路：维护一个尾节点，每次递归时候，将root节点插到尾节点的右子树上，将左子树置为空，为了防止断链，要将左右子树记录下来。

代码如下：

 1 public class Solution {
 2     TreeNode tail = new TreeNode(0);
 3     public void flatten(TreeNode root){
 4         if(root == null) return;
 5         tail.right = root;
 6         tail.left = null;
 7         tail = tail.right;
 8         TreeNode left = root.left;
 9         TreeNode right = root.right;
10         if(root.left != null) flatten(left);
11         if(root.right != null) flatten(right);
12     }
13 }