using System;

public class Program

{

    public static void Main()

    {

        var tree1 = new TreeNode(4) {

         left = new TreeNode(2) {

            left = new TreeNode(1),

            right = new TreeNode(3)

         },

         right = new TreeNode(7){}

        };

​

​

        var result = new Solution().ConvertBST(tree1);

        Console.WriteLine(result);

        Console.WriteLine();

        Console.ReadLine();

    }

}

​

​

​

​

public class TreeNode {

      public int val;

      public TreeNode left;

      public TreeNode right;

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

 }

public class Solution {

    private int MaxValue = 0;

     public TreeNode ConvertBST(TreeNode root) {

      if (root != null)

      {

          ConvertBST(root.right);

          MaxValue += root.val;

          root.val = MaxValue;

          ConvertBST(root.left);

      }

      return root;

 }

}