[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;
    }
}