public static LeetCode538TreeNode ConvertGreaterBST(LeetCode538TreeNode root)
 {
     if (root != null)
     {
         ConvertGreaterBST(root.right);
         sum     += root.val;
         root.val = sum;
         ConvertGreaterBST(root.left);
     }
     return(root);
 }
 public LeetCode538TreeNode(int val = 0, LeetCode538TreeNode left = null, LeetCode538TreeNode right = null)
 {
     this.val   = val;
     this.left  = left;
     this.right = right;
 }