almost lost everything there :/

4/digitSum.c

@@ -0,0 +1,36 @@
+#include <stdio.h>
+
+int calculate_iter(int n)
+{
+    int c = 0;
+    while (n > 0)
+    {
+        c += n % 10;
+        n /= 10;
+    }
+    return c;
+}
+
+int calculate_recur(int n)
+{
+    if (n <= 0)
+        return 0;
+    return n % 10 + calculate_iter(n / 10);
+}
+
+// both complexity O(logn)
+
+int main(int argc, char const *argv[])
+{
+    int w;
+    printf("number: ");
+    scanf("%d", &w);
+    int a = calculate_iter(w);
+    int b = calculate_recur(w);
+    if (a != b)
+        printf("oops! %d != %d\n", a, b);
+    else
+        printf(" yay! %d == %d\n", a, b);
+    // main(argc, argv);
+    return 0;
+}

4/sphenic_recur.c

@@ -0,0 +1,53 @@
+#include <stdio.h>
+#include <stdbool.h>
+
+bool _sphenic(int num, int cnt, int pr)
+{
+    // Base case: If we have already found 3 distinct primes, check if the number is fully divided.
+    if (cnt == 3)
+        return num == 1;
+
+    // Try dividing the number by the next prime factor.
+    while (++pr <= num)
+    {
+        if (num % pr == 0)
+        {
+            num /= pr;
+
+            // If the same prime factor divides it again, it's not sphenic.
+            if (num % pr == 0)
+                return false;
+
+            // Recursively check for the next prime and increase the count of distinct primes.
+            return _sphenic(num, cnt + 1, pr);
+        }
+    }
+
+    // If we run out of primes without finding 3 distinct ones, it's not sphenic.
+    return false;
+}
+
+bool rsphenic(int n)
+{
+    // Sphenic numbers are always greater than or equal to 30.
+    if (n < 30)
+        return false;
+
+    // Initialize the recursion to start with prime checking from 2.
+    return _sphenic(n, 0, 1);
+}
+
+int main()
+{
+    int N = 200;
+
+    for (int i = 1; i < N; i++)
+    {
+        // printf("%d\n", i);
+
+        if (rsphenic(i))
+            printf("%d ", i);
+    }
+
+    return 0;
+}