Rotate N size list by K to the right in O(N) time, O(1) space.

main.c

#include <stdio.h>

#define N 20
#define K 5

// reverses letters in indices s[from:to]
void reverse_string(char * s, int from, int to)
{
	char * p1 = s + from;
	char * p2 = s + to;
	while(p1 < p2) {
		char c = *p1;
		*p1 = *p2;
		*p2 = c; 
		p1++;
		p2--;
	}
}

int main() {
	char A[N];
	int i;
	
	// fill array with example letters
	for(i=0; i<N; i++) 
		A[i] = i+'a';

	// rotate string right by k
	int amt = N % K;
	reverse_string(A, 0, N-1);
	reverse_string(A, 0, K-1);
	reverse_string(A, K, N-1);

	// print array to show output
	for(i=0; i<N; i++) 
		printf("%c ", A[i]);
	printf("\n");

	return 0;
}