❔ How come shall the data type be int, not TreeNode?

public class Solution {
    public bool IsValidBST(TreeNode root) {
        List<TreeNode> l = new List<TreeNode>();
        for(int i = 0; i < l.Count-1; i++){
            if(l[i] >= l[i+1]){
                return false;
            }

        }
        return Helper(root,l);
    }

    public List<TreeNode> Helper(TreeNode root, List<TreeNode> l){
        List<TreeNode> l = new List<TreeNode>();

        if(root != null){
            
          return l;
        }

            l.Add(Helper(root.left));
            l.Add(root.val);
            l.Add(Helper(root.right));
        return l;

        
    }
}