added weekly exercises of isp-ws25

c/isp-ws25/weeklies/week03/w03ex03.c

@@ -0,0 +1,36 @@
+#include <stdio.h>
+
+int lucas(int number);
+
+int main(void)
+{
+  int index_position = 0;
+  int lucas_number = 0;
+  
+  printf("Enter index position: ");
+  scanf("%d", &index_position);
+  
+  if(index_position < 0)
+  {
+    printf("Entered position must be a non-negative integer\n");
+    return 1;
+  }
+
+  lucas_number = lucas(index_position);
+
+  printf("lucas(%d) = %d\n", index_position, lucas_number);
+  
+  return 0;
+}
+
+int lucas(int number)
+{
+  int result = 0;
+  
+  if(number == 0){return 2;}
+  if(number == 1){return 1;}
+
+  result = lucas(number - 1) + lucas(number - 2);
+  
+  return result;
+}