The most effective approach is deterministic when you create time combinations based on numbers. Since the time has only 4 digits, the maximum combination is 4! = 24. This algorithm creates a maximum of 24 combinations (recursively), but will have a short circuit and only continue building combinations that have valid times. Thus, the actual number of combinations will usually be less than 24.
Given combinations (maximum 24), they are sorted by time. The next time it's just the next item in the sort, following the given time.
Algorithm:
Input: valid time string T
Exit: next time after input time N
Create an empty set (to save the correct times) S
Create valid combinations of T and add to S
Convert S to L
Sort L
Define the index I in L T
If size (L) == 1 then N = T
else N = element in L with index i + 1 (size of the list of modules)
And the code:
public class NextTime { public static void main(String[] args) { String time = "12:12"; System.out.println("Starting time="+time); System.out.println("Next time="+getNextTime(time)); } static Set<Integer> times = new HashSet<>(); private static String getNextTime(String time) { // Create a list with the 4 digits (strip out the ':') List<String> digits = Arrays.asList(time.split("")).stream().filter(d -> !d.equals(":")) .collect(Collectors.toList()); // Build up the set of maximum 24 times timeCombinations(digits, 0, ""); // Sort the list of times (which are integers representing minutes) List<Integer> sortedTimeList = times.stream().sorted().collect(Collectors.toList()); // Find the next index immediately after the current time int newIndex = 0; if (sortedTimeList.size() > 1) { int currentTime = Integer.valueOf(time.substring(0, 2)) * 60 + Integer.valueOf(time.substring(3, 5)); int listPosition = sortedTimeList.indexOf(currentTime); newIndex = (listPosition + 1) % times.size(); } // Turn the time back into a string String newTimeString = String.format("%02d:%02d", sortedTimeList.get(newIndex)/60, sortedTimeList.get(newIndex)%60); return newTimeString; } private static void timeCombinations(List<String> digits, int depth, String result) { if (depth == 4) { // You have a valid time; Convert time string like 1234 to minutes int minutes = Integer.valueOf(result.substring(0, 2)) * 60 + Integer.valueOf(result.substring(2, 4)); times.add(minutes); } else { for (int d = 0; d < digits.size(); d++) { int c = Integer.valueOf(digits.get(d)).intValue(); // Validation rule for the first digit of the hour if (depth == 0 && c > 2) continue; // Validation rule for the first digit of the minutes if (depth == 2 && c > 5) continue; List<String> digitsClone = new ArrayList<>(digits); String resultClone = new String(result) + digitsClone.get(d); // Validation rule for the complete hour if (depth == 1 && Integer.valueOf(resultClone).intValue() > 23) continue; digitsClone.remove(d); timeCombinations(digitsClone, depth + 1, resultClone); } } }
}