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 integernwhich 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".

Solution: 类似subset，只向后看

    public List<String> readBinaryWatch(int num) {
        List<String> ans = new ArrayList<>();
        int[] time = {1, 2, 4, 8, 1, 2, 4, 8, 16, 32};
        helper(0, 0, 0, time, num, ans);
        return ans;
    }
    private void helper(int index, int hour, int minute, int[] time, int num, List<String> ans) {
        if (num == 0) {
            if (minute < 10) {
                ans.add(hour + ":0" + minute);
            } else {
                ans.add(hour + ":" + minute);
            }
            return;
        }
        for (int i = index; i < 10; i++) {
            if (i < 4) {
                if (hour + time[i] < 12) {
                    helper(i + 1, hour + time[i], minute, time, num - 1, ans);
                }
            } else {
                if (minute + time[i] < 60) {
                    helper(i + 1, hour, minute + time[i], time, num - 1, ans);
                }
            }
        }
    }