Bene, siccome non ho nulla da fare, ho deciso di mettermi a risolvere i problemi di Project Euler (non l'avevo mai fatto...), ora sono arrivato al problema 11...
Il fatto è che a me il codice sembra corretto e compila correttamente, ma il risultato è scorretto. Evidentemente salto qualche sequenza, ma non riesco a capire quale!
Testo problema
codice:
#include <malloc.h>
#include <stdio.h>
unsigned long m[20][20] = 
{{ 8,  2, 22, 97, 38, 15,  0, 40,  0, 75,  4,  5,  7, 78, 52, 12, 50, 77, 91,  8},
 {49, 49, 99, 40, 17, 81, 18, 57, 60, 87, 17, 40, 98, 43, 69, 48,  4, 56, 62,  0},
 {81, 49, 31, 73, 55, 79, 14, 29, 93, 71, 40, 67, 53, 88, 30,  3, 49, 13, 36, 65},
 {52, 70, 95, 23,  4, 60, 11, 42, 69, 24, 68, 56,  1, 32, 56, 71, 37,  2, 36, 91},
 {22, 31, 16, 71, 51, 67, 63, 89, 41, 92, 36, 54, 22, 40, 40, 28, 66, 33, 13, 80},
 {24, 47, 32, 60, 99,  3, 45,  2, 44, 75, 33, 53, 78, 36, 84, 20, 35, 17, 12, 50},
 {32, 98, 81, 28, 64, 23, 67, 10, 26, 38, 40, 67, 59, 54, 70, 66, 18, 38, 64, 70},
 {67, 26, 20, 68,  2, 62, 12, 20, 95, 63, 94, 39, 63,  8, 40, 91, 66, 49, 94, 21},
 {24, 55, 58,  5, 66, 73, 99, 26, 97, 17, 78, 78, 96, 83, 14, 88, 34, 89, 63, 72},
 {21, 36, 23,  9, 75,  0, 76, 44, 20, 45, 35, 14,  0, 61, 33, 97, 34, 31, 33, 95},
 {78, 17, 53, 28, 22, 75, 31, 67, 15, 94,  3, 80,  4, 62, 16, 14,  9, 53, 56, 92},
 {16, 39,  5, 42, 96, 35, 31, 47, 55, 58, 88, 24,  0, 17, 54, 24, 36, 29, 85, 57},
 {86, 56,  0, 48, 35, 71, 89,  7,  5, 44, 44, 37, 44, 60, 21, 58, 51, 54, 17, 58},
 {19, 80, 81, 68,  5, 94, 47, 69, 28, 73, 92, 13, 86, 52, 17, 77,  4, 89, 55, 40},
 { 4, 52,  8, 83, 97, 35, 99, 16,  7, 97, 57, 32, 16, 26, 26, 79, 33, 27, 98, 66},
 {88, 36, 68, 87, 57, 62, 20, 72,  3, 46, 33, 67, 46, 55, 12, 32, 63, 93, 53, 69},
 { 4, 42, 16, 73, 38, 25, 39, 11, 24, 94, 72, 18,  8, 46, 29, 32, 40, 62, 76, 36},
 {20, 69, 36, 41, 72, 30, 23, 88, 34, 62, 99, 69, 82, 67, 59, 85, 74,  4, 36, 16},
 {20, 73, 35, 29, 78, 31, 90,  1, 74, 31, 49, 71, 48, 86, 81, 16, 23, 57,  5, 54},
 { 1, 70, 54, 71, 83, 51, 54, 69, 16, 92, 33, 48, 61, 43, 52,  1, 89, 19, 67, 48}};

struct Lista_s {
	unsigned long v;
	struct Lista_s *next;
};

typedef struct Lista_s Lista_t;
typedef Lista_t *Lista;

Lista insert(Lista a, unsigned long v) {
	Lista q = (Lista)malloc(sizeof(Lista_t));
	q->v = v;
	q->next = a;
	return q;
}

void delete(Lista a) {
	if(a == NULL)
		return;
	delete(a->next);
	free(a);
}

unsigned long maximum(Lista a, unsigned long max) {
	if(a == NULL)
		return max;
	if((a->v) > max)
		return maximum(a->next, a->v);
	return maximum(a->next, max);
}

int main() {
	Lista a = NULL;
	int i, j, k;
	unsigned long temp;
	//calcolo prodotti su righe
	for(i = 0; i < 20; i++)
		for(j = 0; j < 17; j++) {
			temp = 1;
			for(k = 0; k < 4; k++)
				temp *= m[i][j+k];
			a = insert(a, temp);
		}
	//calcolo prodotti su colonne
	for(i = 0; i < 17; i++)
		for(j = 0; j < 20; j++) {
			temp = 1;
			for(k = 0; k < 4; k++)
				temp *= m[i+k][j];
			a = insert(a, temp);
		}
	//calcolo prodotti su diagonali da sinistra a destra
	for(i = 0; i < 17; i++)
		for(j = 0; j < 17; j++) {
			temp = 1;
			for(k = 0; k < 4; k++)
				temp *= m[i+k][j+k];
			a = insert(a, temp);
		}
	//calcolo prodotti su diagonali da destra a sinistra
	for(i = 3; i < 20; i++)
		for(j = 3; j < 20; j++) {
			temp = 1;
			for(k = 0; k < 4; k++)
				temp *= m[i-k][j-k];
			a = insert(a, temp);
		}
  printf("%lu\n", maximum(a, 0));
  delete(a);
  return 0;
}