gistfile1.txt

#include <stdio.h>

void swap(int* a, int* b) {
  int tmp;
  tmp = *a;
  *a = *b;
  *b = tmp;
}

int euclid(int a, int b) {
  int r;

  if(a < b) {
    swap(&a, &b);
  }

  while(b != 0) {
    r = a % b;
    a = b;
    b = r;
  }

  return a;
}

int main(int argc, char** argv) {
  int a, b, answer;
  printf("input a: ");
  scanf("%d", &a);
  printf("input b: ");
  scanf("%d", &b);

  answer = euclid(a, b);
  printf("answer : %d\n", answer);
  return 0;
}

gistfile2.txt

#include <stdio.h>

int euclid(int a, int b) {
  return (b == 0) ? a : euclid(b, a % b);
}

int main(int argc, char** argv) {
  int a, b, answer;
  printf("input a: ");
  scanf("%d", &a);
  printf("input b: ");
  scanf("%d", &b);

  answer = euclid(a, b);
  printf("answer : %d\n", answer);
  return 0;
}