Binary Watch

A binary watch has 4 LEDs on the top which represent the hours (0-11), and the 6 LEDs on the bottom represent the minutes (0-59).

Each LED represents a zero or one, with the least significant bit on the right.

For example, the above binary watch reads “3:25”.

Given a non-negative integer n which represents the number of LEDs that are currently on, return all possible times the watch could represent.

Example:

Input: n = 1
Return: ["1:00", "2:00", "4:00", "8:00", "0:01", "0:02", "0:04", "0:08", "0:16", "0:32"]
Note:
The order of output does not matter.
The hour must not contain a leading zero, for example "01:00" is not valid, it should be "1:00".
The minute must be consist of two digits and may contain a leading zero, for example "10:2" is not valid, it should be "10:02".

Binary Watch Solution

public class BinaryWatch {
    public List<String> readBinaryWatch(int num) {
        ArrayList<String> allTimes = new ArrayList<String>();

        //iterate through all possible time combinations
        for(int i = 0; i < 12; i++) {
            for(int j = 0; j < 60; j++) {
                //if the current number and n have the same number of bits the time is possible
                if(Integer.bitCount(i * 64 + j) == num) {
                    //add the current time to all times arraylist
                    allTimes.add(String.format("%d:%02d", i, j));
                }
            }
        }

        return allTimes;
    }
}