/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public void flatten(TreeNode root) {
        
        if(root == null) 
           return;
   
        var left = root.left;
        var right = root.right;

        flatten(root.left);
        flatten(root.right);

       root.left = null;
       root.right = left;

        var p = root;
        while (p.right != null) {
            p = p.right;
        }
        p.right = right;


    }
}