#include <iostream>

//--------------------------------------------------------------------------
// Data Definition
//-----------------
// A list of digits is either
//  - an empty list
// or a structure consisting of 
//  - first as digit
//  - rest as list of digits
//--------------------------------------------------------------------------
struct ListOfDigit
{
    int         first;
    ListOfDigit *rest;
};

//--------------------------------------------------------------------------
// Data example
//--------------------------------------------------------------------------
ListOfDigit *empty = 0;
ListOfDigit L1 = { 1, empty };
ListOfDigit L2 = { 4, &L1 };
ListOfDigit L3 = { 7, &L2 };

//--------------------------------------------------------------------------
// Code Pattern
//--------------------------------------------------------------------------
//... F(ListOfDigit * L)
//{
//    if (L == 0)
//    {
//        ...
//    }
//    else
//    {
//        ... L->first ...
//        ... F(L->rest) ...
//    }
//}

//--------------------------------------------------------------------------
// Prototypes
//--------------------------------------------------------------------------
int list2num(ListOfDigit *);
int list2num_A(ListOfDigit * L, int multiplier, int acc);

//--------------------------------------------------------------------------
// main
//--------------------------------------------------------------------------
int main()
{
    std::cout << "list2num(empty) == " << list2num(empty) << "\n";
    std::cout << "list2num(&L1) == " << list2num(&L1) << "\n";
    std::cout << "list2num(&L2) == " << list2num(&L2) << "\n";
    std::cout << "list2num(&L3) == " << list2num(&L3) << "\n";

    std::cout << "list2num_A(empty, 10, 2) == " << list2num_A(empty, 10, 2) << "\n";
    std::cout << "list2num_A(&L1, 10, 9) == " << list2num_A(&L1, 10, 9) << "\n";
    std::cout << "list2num_A(&L2, 10, 7) == " << list2num_A(&L2, 10, 7) << "\n";

    return 0;
}

//--------------------------------------------------------------------------
// @description convert a list of digits to a number
// @param L a list of digits
// @return the corresponding number
// @contract list2num : ListOfDigit -> number
// @example list2num(empty) == 0
// @example list2num(L1) == 1
// @example list2num(L2) == 14
// @example list2num(L3) == 147
//--------------------------------------------------------------------------
int list2num(ListOfDigit * L)
{
    if (L == 0)
    {
        return 0;
    }
    else
    {
        return list2num_A(L->rest, 10, L->first);
    }
}

//--------------------------------------------------------------------------
// @description convert a list of digits to a number
// @param L a list of digits
// @param multiplier the current multiplier
// @param acc the accumulated number so far
// @return the corresponding number
// @contract list2num_A : ListOfDigit number number -> number
// @example list2num_A(empty, 10, 2) == 2
// @example list2num_A(L1, 10, 9) == 19
// @example list2num_A(L2, 10, 7) == 147
//--------------------------------------------------------------------------
int list2num_A(ListOfDigit * L, int multiplier, int acc)
{
    if (L == 0)
    {
        return acc;
    }
    else
    {
        return list2num_A(L->rest, multiplier*10, L->first * multiplier + acc);
    }
}