/**
 * 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 {
    TreeNode prev;
    public void flatten(TreeNode root) {
        rec(root);
    }
    private TreeNode rec(TreeNode root){
        if(root == null) return null;
        TreeNode leftNode = root.left;
        TreeNode rightNode = root.right;
        if (prev != null) {
            prev.right = root;
            prev.left = null;
        }
        prev = root;
        TreeNode l = rec(leftNode);
        TreeNode r = rec(rightNode);
        return root;
    }
}