Binary Watch — Python

# Time:  O(1)
# Space: O(1)


class Solution(object):
    def readBinaryWatch(self, num):
        """
        :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 xrange(12) for m in xrange(60)
                if bit_count(h) + bit_count(m) == num]

    def readBinaryWatch2(self, num):
        """
        :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]