Find maximum possible time HH:MM by permuting four given digits

Question

I recently took a coding test for a promotion at work. This was one of the tasks I really struggled with and was wondering what is the best way to do this. I used a load of

Answer 1

Hmmm..... I guess it's really simple if you break it into simpler problems: eg find all valid hours (00-23), for each of these valid hours use the remaining numbers to find valid minutes (00-59), combine and sort. In pseudo code something like the following

    valid_times = []
    function get_max(digits[]) {
                for each d1 in digits[]
            for each d2 in (digits[] except d1)
                res = is_valid_hour(d1, d2)
                if(res > 0) {
                    if(res == 2)
                        swap(d1, d2)
                    d3 = one of the rest in (digits except d1 and d2)
                    d4 = digit left in digits[]
                    res = is_valid_minute(d3, d4)
                    if(res > 0)
                        if(res == 2)
                            swap(d3, d4)
                        add (d1, d2, d3, d4) to valid_times;
                }
        sort(valid_times)
        print valid_times[0]
    }

    function is_valid_hour(a, b) {
        if (a*10+b<24)
            return 1

        if (b*10+a<24)
            return 2

        return 0;
    }

    function is_valid_minute(a, b) {
        if (a*10+b<60)
            return 1

        if (b*10+a<60)
            return 2

        return 0;
    }