update 3 to the right solution

pset3/3-sum-problem.c

@@ -1,27 +0,0 @@
-// 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);
-
-  // Print values
-  printf("%lld\n", sum(num));
-
-  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);
-  }
-}
-

pset3/3-sum.c

@@ -1,24 +1,31 @@
+// 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
-  long long sum = 0;
-  long long n;
-  scanf("%lld", &n);
 
-  // if n is smaller than 3
+  // Initialize variables and get input from user.
-  if (n < 3) {
+  unsigned long long int num;
-    printf("0\n");
+  scanf("%lld", &num);
-    return 0;
+
+  // 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);
 	}
 
-  // Iterate through all
+  printf("%lld\n", num1);
-  for (long long i = 3; i <= n; i++) {
-    sum += i * (i - 1) * (i - 2);
-  }
-
-  // Print the result
-  printf("%lld\n", sum);
-
   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);
+  }
+}
+