/**

 * Definition for a binary tree node.

 * public class TreeNode {

 *     public int val;

 *     public TreeNode left;

 *     public TreeNode right;

 *     public TreeNode(int x) { val = x; }

 * }

 */

public class Solution {

    Stack<int> S = new Stack<int>();

        private void preNode(TreeNode node)

        {

            if (node != null)

            {

                S.Push(node.val);

                if (node.left != null)

                {

                    preNode(node.left);

                }

                if (node.right != null)

                {

                    preNode(node.right);

                }

            }

        }

        public IList<int> PreorderTraversal(TreeNode root)

        {

            preNode(root);

            var list = S.Reverse().ToList();

            return list;

        }

}