divide et impera factorial

factorial-dei.c

#include <stdio.h>

//varianta iterativa
int fact(int n) {
    int p = 1;
    for(int i = 1; i <= n; ++i) {
        p *= i;
    }
    return p;
}

//1,2,3,4,5
//1*2*3*4*5
//[1,2,3] [4,5]
//[1,2],[3] [4][5]
void factorial_divide_et_impera(int lo, int hi, int *res) {

      int a, b;

      if(lo == hi) {
        *res = lo;
      } else {
         int m = (lo + hi) / 2;
         factorial_divide_et_impera(lo, m, &a);
         factorial_divide_et_impera(m + 1, hi, &b);
         *res = a * b;
      }
}

int main(int argc, char const *argv[]) {

  int n = 5;

  //5! = 1 * 2 * 3 * 4 * 5
  int res;

  printf("5! = %d (varianta iterative clasica)\n", fact(n));

  factorial_divide_et_impera(1, n, &res);

  printf("5! = %d (divide et impera)", res);

  return 0;
}