add problem 3

pset3/3-sum-problem.c

@@ -0,0 +1,27 @@
+// 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

@@ -0,0 +1,24 @@
+#include <stdio.h>
+
+int main(void) {
+  // Initialize variables and get input
+  long long sum = 0;
+  long long n;
+  scanf("%lld", &n);
+
+  // if n is smaller than 3
+  if (n < 3) {
+    printf("0");
+    return 0;
+  }
+
+  // Iterate through all
+  for (long long i = 3; i < n; i++) {
+    sum += i * (i - 1) * (i - 2);
+  }
+
+  // Print the result
+  printf("%lld\n", sum);
+
+  return 0;
+}