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 Queue<TreeNode> queue;
    
    public void flatten(TreeNode root) {
        queue = new LinkedList<>();
        preorder(root);
        
        TreeNode node = null;
        
        while (!queue.isEmpty())
        {
            if (node == null)
            {
                node = queue.poll();
                node.left = null;
            }
            else
            {
                node.right = queue.poll();
                node.left = null;
                node = node.right;
            }
        }
    }
    
    private void preorder(TreeNode root)
    {
        if (root == null)
            return;
            
        queue.offer(root);
        preorder(root.left);
        preorder(root.right);
    }
}