[Leetcode] 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 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) {        flattenHelper(root);    }    private TreeNode flattenHelper(TreeNode node) {        if(node == null) {            return null;        }        TreeNode right = node.right;        node.right = node.left;        TreeNode leftLastNode = flattenHelper(node.left);        node.left = null;        if(leftLastNode == null) {            leftLastNode = node;        }        leftLastNode.right = right;        TreeNode rightLastNode = flattenHelper(leftLastNode.right);        if(rightLastNode == null) {            return leftLastNode;        }        return rightLastNode;    }}