My solution to Facebook Programming Challenge sample problem: Missing number in arithmetic sequence. Source: http://retina.pvey.es/solving-facebook-programming-challenge-sample-problem

fb-arithmetic.c

#include <stdio.h>

int main() {
	int input, val;
	int init, delta = 0, newdelta;
	int i;

	scanf("%d\n", &input);
	scanf("%d ", &init);
	
	for (i = 1; i < input; i++) {
		scanf("%d ", &val);
		newdelta = val - init;

		if (newdelta == 2 * delta) {
			// skip on current loop
			printf("%d\n", val - delta);
			break;
		}
		else if (delta == 2 * newdelta) {
			// skip on previous loop
			printf("%d\n", init - newdelta);
			break;
		}

		delta = newdelta;
		init = val;
	}

	return 0;
}