public class Solution
{
    public void Flatten(TreeNode? root)
    {
        if (root == null)
            return;
        var head = root;
        var stack = new Stack<TreeNode>();
        if (root.right != null)
            stack.Push(root.right);
        if (root.left != null)
            stack.Push(root.left);
        while (stack.Count > 0)
        {
            var node = stack.Pop();
            head.left = null;
            head.right = node;
            head = node;
            if (node.right != null)
                stack.Push(node.right);
            if (node.left != null)
                stack.Push(node.left);

        }
    }
}