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

非递归：

public class Solution {    public void flatten(TreeNode root) {        TreeNode now = root;        while(now != null){            if(now.left != null){                TreeNode pre = now.left;                while(pre.right != null){                    pre = pre.right;                }                pre.right = now.right;                now.right = now.left;                now.left = null;            }            now = now.right;        }    }}

递归：

public class Solution {    TreeNode pre = null;    public void flatten(TreeNode root) {        if(root == null) return;        flatten(root.right);        flatten(root.left);        root.right = pre;        root.left = null;        pre = root;    }}