BUPT-homework/semester1/pset3/3-sum.c

// Try to use recursion to solve this problem.
#include <stdio.h>

// function prototypes.

unsigned long long int sum(unsigned long long int n);

int main(void) {

  // Initialize variables and get input from user.
  unsigned long long int num;
  scanf("%lld", &num);

  // Calculate the sum of the numbers. (Can be optimized.)
	unsigned long long int num1 = 0;
	for (int i = 1; i <= num; i++) {
		num1 += sum(i);
	}

  printf("%lld\n", num1);
  return 0;
}

unsigned long long int sum(unsigned long long int n) {
  if (n == 1 || n == 2) { // Case 1: n is 1 or 2
    return 0;
  } else { // Case 2: n is not
    return n * (n - 1) * (n - 2) + sum(n - 1);
  }
}