114. Flatten Binary Tree to Linked List

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

For example,
Given

The flattened tree should look like:

遍历的时候根右左

逆向的前序遍历

 1 class Solution {
 2     public void flatten(TreeNode root) {
 3         root =help(root,null);;
 4         
 5     }
 6     private TreeNode help(TreeNode root,TreeNode prev){
 7         if(root==null) return prev;
 8         //记录下上次访问的节点，
 9         //上次访问的节点就是当前节点的右子树，左子树是null
10         prev = help(root.right,prev);
11         prev = help(root.left,prev);
12         root.right = prev;
13         root.left = null;
14         
15         return root;
16         
17     }
18 }