Here's my solution in Java:

public static String timeConversion(String s) {
    String meridiem = s.substring(8);
    int hours = Integer.parseInt(s.substring(0, 2));
    String colonsMinutesAndSeconds = s.substring(2, 8);

    if (meridiem.equals("AM") && hours == 12) {
        hours = 0;
    } else if (meridiem.equals("PM") && hours != 12) {
        hours += 12;
    }

    return String.format("%02d%s", hours, colonsMinutesAndSeconds);
}