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".

public class Solution {
    public IList<string> ReadBinaryWatch(int num) {
      var output = new List<string>();
      for(int h = 0; h <= 11; h++){
        for(int m = 0 ; m <= 59; m++){
          char[] charlist = (Convert.ToString(h,2) + Convert.ToString(m,2)).ToCharArray();
          var count = charlist.Count(c => c == '1');
          if(count == num){
            output.Add(string.Format("{0}:{1:00}", h, m));
          }
        }
      }
      return output;
    }
}