hackerrank-cipher.c

//https://www.hackerrank.com/challenges/cipher

#include <stdio.h>
#include <string.h>
#include <math.h>
#include <stdlib.h>

char xor(char a, char b) {
    int ai = a-'0';
    int bi = b-'0';
    return (ai ^ bi) + '0';
}

int main() {
    int n, k;
    char s[2000005];
    scanf("%d", &n);
    scanf("%d", &k);
    scanf("%s", s);

    char sol[n+1];
    sol[0] = s[0];
    for (int i = 1; i < k; ++i) {
        sol[i] = xor(s[i-1], s[i]);
    }
    for (int i = k; i < n; ++i) {
        sol[i] = xor(xor(s[i-1], sol[i-k]), s[i]);
    }
    sol[n] = '\0';
    printf("%s\n", sol);
    return 0;
}

Elegant solution