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).

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”.

This problem requires me to think the other way round.
I began thinking about given all the bits combination and then output all the times.

But thesmarter way is to give out all the time compositions that match the bin numbers required.
To do this, we use Integer.bitCount() to calculate the count of “1” is the binaray representation of an Integer. For example, “12” is represented as “1100”, so the bit count (or say, the population) is 2.

And to format a string, we new a String by using String.format("%05d", yournumber);
The "%05d" represents how many zeroes should be used as padding.

class Solution {
    public List<String> readBinaryWatch(int num) {
        List<String> res = new LinkedList<>();
        for(int i=0;i<12;i++){
            for(int j=0;j<60;j++){
                int n = Integer.bitCount(i)+Integer.bitCount(j);
                if(n==num) res.add(String.format("%d:%02d",i,j));
            }
        }
        return res;
    }
}