401. Binary Watch (Easy)

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".

Solution:

vector<string> readBinaryWatch(int num) {
    vector<string> res;

    for (int i = 0; i < 12; ++i) {
        for (int j = 0; j < 60; ++j) {
            if (bitset<4>(i).count() + bitset<6>(j).count() == num) {
                string minute = to_string(j);
                res.push_back(to_string(i)+":"+(minute.size()<2 ? "0":"")+minute);
            }
        }
    }

    return res;
}