[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

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