Binary Watch

Time: O(1); Space: O(1); easy

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

Output: [“1:00”, “2:00”, “4:00”, “8:00”, “0:01”, “0:02”, “0:04”, “0:08”, “0:16”, “0:32”]

Notes:

  • 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”.

[1]:

class Solution1(object):
    def readBinaryWatch(self, num) -> list:
        """
        :type num: int
        :rtype: List[str]
        """
        def bit_count(bits):
            count = 0
            while bits:
                bits &= bits-1
                count += 1
            return count

        return ['%d:%02d' % (h, m)
                for h in range(12)
                for m in range(60)
                if bit_count(h) + bit_count(m) == num]

[2]:

s = Solution1()
assert s.readBinaryWatch(1) == ['0:01', '0:02', '0:04', '0:08', '0:16', '0:32', '1:00', '2:00', '4:00', '8:00']

[4]:

class Solution2(object):
    def readBinaryWatch(self, num) -> list:
        """
        :type num: int
        :rtype: List[str]
        """
        return ['{0}:{1}'.format(str(h), str(m).zfill(2))
                for h in range(12)
                for m in range(60)
                if (bin(h) + bin(m)).count('1') == num]

[5]:

s = Solution2()
assert s.readBinaryWatch(1) == ['0:01', '0:02', '0:04', '0:08', '0:16', '0:32', '1:00', '2:00', '4:00', '8:00']