abstract class Tree
    {
        ExpressionTreeNode root;
    }

    class leaf extends Tree
    {
        String numbervalue;

        public leaf(String expr)
        {
            this.numbervalue = expr;
        }
    }


    class Expr extends Tree
    {
        Tree left, right;
        String operation;
        List<String> listofstrings;

        public Expr(String operation, Tree left, Tree right, List<String> stringlist)
        {
            this.operation = operation;
            this.left = left;
            this.right = right;
            this.listofstrings = stringlist;
        }
    }

    List<String> listofstrings; // input

    public Tree makeBinaryTree(List<String> listofstrings)
    {
        String element = listofstrings.get(0);
        Tree left, right;
        //Expr newexpr = new Expr(element, left, right, listofstrings);

        if(isanumber(element))
        {
            return new leaf(element);
        }
        else
        {
            left = makeBinaryTree(listofstrings);
            right = makeBinaryTree(listofstrings);
        }
        listofstrings = removefirst(listofstrings);
        return new Expr(element, left, right, listofstrings);
    }