401. 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".

# @param {Integer} num
# @return {String[]}
def read_binary_watch(num)
  output = []
  (0..11).each do |h|
    (0..59).each do |m|
      if (h.to_s(2) + m.to_s(2)).count('1') == num
        output << "#{h}:#{sprintf("%02d",m)}"
      end
    end
  end
  output
end