401. 二进制手表 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 = 1Return: ["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".

二进制手表上的数字有一个特点：转换成二进制后，只有1位的数字为1

例如： 2=10

4=100

8=1000

可以利用这一特性进行解题

static public List<string> ReadBinaryWatch(int num) {
	List<string> list = new List<string>();
	string s = "";
	for (int i = 0; i < 12; i++) {
		for (int j = 0; j < 60; j++) {
			if (BitCount(i) + BitCount(j) == num) {
				list.Add(i.ToString() + ":" + NumberToString(j));
			}
		}
	}
	return list;
}

static public int BitCount(int number) {
	int sum = 0;
	while (number > 0) {
		if ((number & 1) == 1) {
			sum++;
		}
		number >>= 1;
	}
	return sum;
}

static public string NumberToString(int number) {
	string s = number.ToString();
	return s.PadLeft(2, ‘0‘);
}

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