sum.c

#include <stdio.h>

int sum0(int n)
{
  return n > 0 ? sum0(n - 1) + n : 0;
}

int sum1(int n)
{
  return n > 0 ? sum1(n / 10) + (n % 10) : 0;
}

int main ()
{
  printf("sum0 = %d\n", sum0(1234)); // 0 + 1 + 2 + 3 + 4 + 5...n
  printf("sum1 = %d\n", sum1(1234)); // 0 + 1 + 2 + 3 + 4
  return 0;
}