All binary strings of length n containing exactly k ones.

bits.c

/*
// Poor man’s C++ solution.

#include<algorithm>
#include<string>
#include<iostream>

int main() {
	int n, k;
	std::cin >> n >> k;

	std::string s(n, '1');
	std::fill_n(s.begin(), n - k, '0');
	do {
		std::cout << s << std::endl;
	} while (std::next_permutation(s.begin(), s.end()));
}
*/

// https://graphics.stanford.edu/~seander/bithacks.html
//
// But imagine we had 128 bit integers…
// Oh, wait.
//
typedef unsigned __int128 u128;

#ifdef __cplusplus
extern "C" {
	int scanf(const char* fmt, ...);
	int putchar(int c);
	int puts(const char* s);
}
#else
int scanf(const char* restrict fmt, ...);
int putchar(int c);
int puts(const char* s);
#endif

u128 reverse(u128 v) {
	u128 s = 128;
	u128 mask = ~0;
	while((s >>= 1) > 0) {
		mask ^= (mask << s);
		v = ((v >> s) & mask) | ((v << s) & ~mask);
	}
	return v;
}

void print(u128 v, int n) {
	v <<= (128 - n);
	v = reverse(v);
	for(; n > 0; --n) {
		if (v & 1)
			putchar('1');
		else
			putchar('0');
		v >>= 1;
	}
	puts("");
}

u128 next(u128 v) {
	u128 t = (v | (v - 1)) + 1;  
	u128 w = t | ((((t & -t) / (v & -v)) >> 1) - 1);  
	return w;
}

int main() {
	int n, k;
	scanf("%d%d", &n, &k);
	if(100 < n || n < k) {
		puts("assertion failure");
		return 1;
	}

	if(n <= 0) {
		return 0;
	}
	if(k <= 0) {
		print(0, n);
		return 0;
	}

	u128 w = ~0;
	w >>= (128 - k);

	u128 o = 1;
	o <<= n;
	while(w < o) {
		print(w, n);
		w = next(w);
	}
	return 0;
}