using System;

public class Solution

{

    public static void Main()

    {

        var n1 = new TreeNode(1, new TreeNode(2, new TreeNode(3), new TreeNode(4)), new TreeNode(5, null, new TreeNode(6)));

​

        Flatten(n1);

​

        PreOrderTraversal(n1);

    }

    static void PreOrderTraversal(TreeNode node) {

        Console.Write("{0} -> ", node.val);

        if (node.left != null) PreOrderTraversal(node.left);

        if (node.right != null) PreOrderTraversal(node.right);

    }

    static void Flatten(TreeNode root) {

        // Handle for leaf nodes

        if (root == null || (root.left == null && root.right == null)) return;

​

        if (root.left != null) {

            var tmp = root.right;

            root.right = root.left;

            root.left = null;

​

            var curr = root;

            while(curr.right != null) {

                curr = curr.right;

            }

            curr.right = tmp;

​

        }

​

        if (root.right != null) Flatten(root.right);

    }

}

​

public class TreeNode {

     public int val;

     public TreeNode left;

    public TreeNode right;

     public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {

         this.val = val;

         this.left = left;

         this.right = right;

     }

}

​