Flatten Binary Tree to Linked List

A very beautiful tree traversal problem. No doubt recursion will be the solution.
Track the last node visited, and make the current node as the right child of the last node. Recursively go down to the left, then go to the right.

  public class TreeNode {
      int val;
      TreeNode left;
      TreeNode right;
      TreeNode(int x) { val = x; }
  }
 
public class FlattenTree {
    private TreeNode lastNode = null;
    public void flatten(TreeNode root) {
        if (root == null)
            return;
        if (lastNode != null)
        {
            lastNode.left = null;
            lastNode.right = root;
        }
        lastNode = root;
        TreeNode right = root.right;
        flatten(root.left);
        flatten(right);
            
        
    }
}