From 9394b51158220c27e18830beaccbbe16edfc307f Mon Sep 17 00:00:00 2001
From: Simon Wiesend <wiesendsi102436@th-nuernberg.de>
Date: Sun, 7 Dec 2025 17:48:50 +0100
Subject: [PATCH] fix: getDuplicate() must not modify const numbers array

---
 numbers.c | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/numbers.c b/numbers.c
index 5211acb..ef96dc2 100644
--- a/numbers.c
+++ b/numbers.c
@@ -74,13 +74,27 @@ unsigned int *createNumbers(unsigned int len)
 // Returns only the only number in numbers which is present twice. Returns zero on errors.
 unsigned int getDuplicate(const unsigned int numbers[], unsigned int len)
 {
-  qsort((void *)numbers, len, sizeof(int), compareInt); // sort the array
+  unsigned int *numbersCpy = malloc(sizeof(unsigned int) * len);
+  if (!numbersCpy)
+  {
+    return 0;
+  }
+  memcpy(numbersCpy, numbers, len * sizeof(unsigned int));
+  numbersCpy = numbersCpy; // shadow the numbers array with copy
+
+  qsort((void *)numbersCpy, len, sizeof(int), compareInt); // sort the array
+
+  unsigned int duplicateFound = 0; // zero on errors
   for (int i = 0; i < len - 1; i++)
   {
-    if (numbers[i] == numbers[i + 1])
-      return numbers[i];
+    if (numbersCpy[i] == numbersCpy[i + 1])
+    {
+      duplicateFound = numbersCpy[i];
+      break;
+    }
   }
-  return 0; // zero on errors
+  free(numbersCpy);
+  return duplicateFound;
 }
 
 static int compareInt(const void *ptr1, const void *ptr2)