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".

題目含義：就是用二進制的方式實現的手錶，Led燈來表示二進制，亮着表示置位1，不亮表示0。4個位表示小時，6個位表示分鐘。題目要求給定亮着的Led燈個數，求出全部能夠表示的時間列表。

 

1     public List<String> readBinaryWatch(int num) {
2         List<String> times = new ArrayList<>();
3         for (int h = 0; h < 12; h++)
4             for (int m = 0; m < 60; m++)
5                 if (Integer.bitCount(h) + Integer.bitCount(m) == num)
6                     times.add(String.format("%d:%02d", h, m));
7         return times;        
8     }