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

For example, given the following tree:

1   / \  2   5 / \   \3   4   6

The flattened tree should look like:

1 \  2   \    3     \      4       \        5         \          6

难度：medium

题目：给定二叉树，原地扁平化。

思路：后序遍历

Runtime: 6 ms, faster than 100.00% of Java online submissions for Flatten Binary Tree to Linked List.

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */class Solution {    public void flatten(TreeNode root) {        rightFlatten(root);    }        private TreeNode rightFlatten(TreeNode root) {        if (null == root) {            return root;        }        TreeNode left = rightFlatten(root.left);        TreeNode right = rightFlatten(root.right);        if (left != null) {            root.left = null;            root.right = left;            TreeNode rightMost = left;            while (rightMost.right != null) {                rightMost = rightMost.right;            }            rightMost.right = right;        }                return root;    }}