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".

题目含义：就是用二进制的方式实现的手表，Led灯来表示二进制，亮着表示置位1，不亮表示0。4个位表示小时，6个位表示分钟。题目要求给定亮着的Led灯个数，求出所有可以表示的时间列表。

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