_401.java

package com.fishercoder.solutions;

import java.util.ArrayList;
import java.util.List;

/**
 * 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".
 */
public class _401 {

    public List<String> readBinaryWatch(int num) {
        List<String> times = new ArrayList<>();
        for (int h = 0; h < 12; h++) {
            for (int m = 0; m < 60; m++) {
                if (Integer.bitCount(h * 60 + m) == num) {
                    times.add(String.format("%d:%02d", h, m));//%02 means to pad this two-digit decimal number on the left with zeroes
                }
            }
        }
        return times;
    }

}