solution2.java

import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import java.time.temporal.ChronoUnit;
import java.util.*;

public class Solution {

    private static boolean shouldUserBeAsked(List<String> dates) {
        Collections.reverse(dates);
        DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
        LocalDateTime lastDateTime = LocalDateTime.parse(dates.get(0), formatter);

        // Start with ones because there will always be at least one-day-sequence and at least one session.
        int numberOfSessions = 1;
        int daysInARow = 1;

        for (String s : dates) {
            LocalDateTime dateTime = LocalDateTime.parse(s, formatter);

            // Calculate time difference in minutes.
            if (dateTime.until(lastDateTime, ChronoUnit.MINUTES) > 30)
                numberOfSessions += 1;

            // Using day of the week because of the constant modulo.
            int daysDifference = (lastDateTime.getDayOfWeek().getValue() - dateTime.getDayOfWeek().getValue()) % 7;
            if (daysDifference == 1)
                daysInARow += 1;

            // User broke sequence so we can just stop.
            if (daysDifference > 1)
                break;

            lastDateTime = dateTime;
        }
        return numberOfSessions >= 6 && daysInARow >= 3;
    }

    public static void main(String[] args) {
        String[] A = new String[] { "2017-03-10 08:13:11", "2017-03-10 19:01:27", "2017-03-11 07:35:55", "2017-03-11 16:15:11", "2017-03-12 08:01:41", "2017-03-12 17:19:08" };
        List<String> a =  new ArrayList<>(Arrays.asList(A));

        String[] B = new String[] { "2017-03-10 18:58:11", "2017-03-10 19:01:27", "2017-03-11 07:35:55", "2017-03-11 16:15:11", "2017-03-12 08:01:41", "2017-03-12 17:19:08" };
        List<String> b =  new ArrayList<>(Arrays.asList(B));

        String[] C = new String[] { "2017-03-08 17:11:13", "2017-03-11 17:22:47", "2017-03-11 19:23:51", "2017-03-11 22:03:12", "2017-03-12 08:32:04", "2017-03-12 13:19:08", "2017-03-12 17:19:08" };
        List<String> c =  new ArrayList<>(Arrays.asList(C));

        System.out.println(shouldUserBeAsked(a));
        System.out.println(shouldUserBeAsked(b));
        System.out.println(shouldUserBeAsked(c));

    }
}