Binary numbers with N digits and k 1s

run.swift

// You are trying to figure out how many binary numbers there are that have N digits and k 1s, and are multiples of 3. Binary numbers that have N digits also include ones that start with 0.

// Suppose that N = 3 and K = 2. Binary numbers with three digits in this case include the following: 000, 001, 010, 011, 100, 101, 110, and 111--8 binary numbers in total. Among these numbers, the ones that have two 1s are 011, 101, and 110. Among these three numbers, two--011 and 110--are multiples of 3.

// Suppose parameters N and K are given, where N represents the number of digits of the given binary number(s) and K the number of 1s in these binary numbers. Please write a function solution that returns the number of binary numbers that have N digits and k 1s and are multiples of 3

// Constraints
// N is a natural number between 1 and 50.
// K is a natural number between 1 and N.
// Examples
// N	K	result
// 3	2	2
// 4	2	4
// 6	3	2
// Example #1

// Demonstrated in the prompt above.

// Example #2

// The binary numbers that have two 1s and 4 digits and are multiples of 3 are 0011, 0110, 1001, and 1100--4 in total.

// Example #3

// The binary numbers that have three 1s and 6 digits and are multiples of 3 are 101010 and 010101--two.

// -----

/*
    Swift 4 program for
    Generate all the binary strings of N bits
*/
class BinaryText
{
	func solution(_ record: String, _ start: Int, _ last: Int) -> [String]
	{
		if (start == last)
		{
			return [record];
		}
		// Find result using recursion
		let binaryNumber1 = self.solution(record + String("0"), start + 1, last);
		let binaryNumber2 = self.solution(record + String("1"), start + 1, last);
        return binaryNumber1 + binaryNumber2;
	}
	func binaryString(_ n: Int) -> [String]
	{
		// N indicate digit in binary
		if (n <= 0)
		{
			return [];
		}
		// print(" Digit : ", n ," ");
		let binaryNumber: [String] = self.solution("", 0, n);
        return binaryNumber
	}
}
func main(_ n: Int,_ k: Int)
{
	let task: BinaryText = BinaryText();
	var array: [String] = task.binaryString(n);
    array = array.filter({
        $0.components(separatedBy:"1").count - 1 == k
    })
    array = array.filter({
        Int($0, radix: 2)! % 3 == 0
    })
    print(array)
}
main(3, 2);