diff -r 34feeb53c0ba -r 0315d9983cd0 core_testing1/collatz.scala
--- a/core_testing1/collatz.scala	Sun Jan 15 10:58:13 2023 +0000
+++ b/core_testing1/collatz.scala	Sat Mar 11 22:01:53 2023 +0000
@@ -1,46 +1,33 @@
+import scala.annotation.tailrec
 // Core Part 1 about the 3n+1 conjecture
 //============================================
 
 object C1 {
 
-// ADD YOUR CODE BELOW
-//======================
-
-// test1 7 Nov
-// test2
-// test3
-// test4
-
+@tailrec
+private def collatz(n: Long, steps: Long = 0): Long = {
+  if (n == 1) steps
+  else if (n % 2 == 0) collatz(n / 2, steps + 1)
+  else collatz(n * 3 + 1, steps + 1)
+}
 
-//(1) 
-def collatz(n: Long) : Long = 
-  if (n == 1) 0 else
-    if (n % 2 == 0) 1 + collatz(n / 2) else 
-      1 + collatz(3 * n + 1)
-
-
-//(2) 
-//def collatz_max(bnd: Long) : (Long, Long) = {
-//  val all = for (i <- (1L to bnd)) yield (collatz(i), i)
-//  all.maxBy(_._1)
-//}
-
-def collatz_max(bnd: Long): (Long, Long) = {
-  val all = for (i <- (1L to bnd)) yield (collatz(i), i)
-  all.maxBy(_._1)
+def collatz_max(upper: Long): (Long, Long) = {
+  (1L to upper).map(n => (collatz(n), n)).maxBy(_._1)
 }
 
 
-
-//(3)
-
-def is_pow_of_two(n: Long) : Boolean = (n & (n - 1)) == 0
+private def is_pow_of_two(n: Long) : Boolean = {
+  (n & (n - 1)) == 0
+}
 
-def is_hard(n: Long) : Boolean = is_pow_of_two(3 * n + 1)
+private def is_hard(n: Long) : Boolean = {
+  is_pow_of_two(3 * n + 1)
+}
 
-def last_odd(n: Long) : Long = if (is_hard(n)) n else
-    if (n % 2 == 0) last_odd(n / 2) else 
-      last_odd(3 * n + 1)
+
+private def last_odd(n: Long): Long = {
+  (1L to n).filter(is_hard).max
+}
 
 }