Get all binary combination of size n but only k 1s

Question

So i'm trying to generate all binaries of a size n but with the condition that only k 1s. i.e

for size n = 4, k=2, (there is 2 over 4 combinations)

1100
1010
1001
0110
0101
0011

I'm stuck and can't figure out how to generate this.

Answer 1

Using the basic recursive method for printing all binary sequence all that remains is to enforce your constraints:

    private static void binSeq(int n, int k, String seq) {
    if (n == 0) {
        System.out.println(seq);
        return;
    }

    if (n > k) {
        binSeq(n - 1, k, seq + "0");
    }

    if (k > 0) {
        binSeq(n - 1, k - 1, seq + "1");
    }
}

Answer 2

Here's my non-recursive take on this algorithm. Because there are 2^n permutations of binary strings, we can use a for-loop to iterate through every possible string and check if the amount of "1"s is not equal to k:

private static void generate(int n, int k) {
    for (int i = 0; i < Math.pow(2, n); i++) {
        if (Integer.bitCount(i) != k) {
            continue;
        }

        String binary = Integer.toBinaryString(i);

        if (binary.length() < n) {
            System.out.format("%0" + (n - binary.length()) + "d%s\n", 0, binary);
        } else {
            System.out.println(binary);
        }
    }
}