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; }
 * }
 */
public class Solution {
	public void flatten(TreeNode root) {
		LinkedList<TreeNode> linkedList = new LinkedList<TreeNode>();
		TreeNode p = root;
		while (p != null || !linkedList.isEmpty()) {
			if (p.right != null) {
				linkedList.addFirst(p.right);
			}
			if (p.left != null) {
				p.right = p.left;
				p.left = null;
			} else if (!linkedList.isEmpty()) {
				p.right = linkedList.poll();
			}
			p = p.right;
		}
	}
}