LeetCode-114 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

Hints:

If you notice carefully in the flattened tree, each node‘s right child points to the next node of a pre-order traversal.

分析：

使用递归，存储原始树的左子树。需要注意的是题目要求in-place,因此不能用new TreeNode(X)构造新节点并作为结果返回。

代码如下：

 1 class TreeNode {
 2     int val;
 3     TreeNode left;
 4     TreeNode right;
 5     TreeNode(int x) { val = x; }
 6 }
 7
 8 public class Solution {
 9     public void flatten(TreeNode root) {
10         if(root != null) {
11             TreeNode left = root.left;
12             TreeNode right = root.right;
13
14             root.left = null;
15             root.right = left;
16             flatten(root.right);
17
18             while(root.right != null) {
19                 root = root.right;
20             }
21
22             root.right = right;
23             flatten(root.right);
24         }
26     }
27 }