Add reverseString program for Hacktoberfest contribution

merge sorted lists

@@ -0,0 +1,32 @@
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     struct ListNode *next;
+ * };
+ */
+
+struct ListNode* mergeTwoLists(struct ListNode* list1, struct ListNode* list2) {
+    // Create a dummy node to simplify merging
+    struct ListNode dummy;
+    struct ListNode* tail = &dummy;
+    dummy.next = NULL;
+
+    // Merge both lists while both are non-empty
+    while (list1 && list2) {
+        if (list1->val < list2->val) {
+            tail->next = list1;
+            list1 = list1->next;
+        } else {
+            tail->next = list2;
+            list2 = list2->next;
+        }
+        tail = tail->next;
+    }
+
+    // Attach the remaining part (if any)
+    if (list1) tail->next = list1;
+    else tail->next = list2;
+
+    return dummy.next;
+}

reverseString.c

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+// reverseString.c
+// Added as part of Hacktoberfest 2025 contribution
+// Simple program to reverse a string in C
+
+#include <stdio.h>
+#include <string.h>
+
+void reverseString(char *str) {
+    int n = strlen(str);
+    for (int i = 0; i < n / 2; i++) {
+        char temp = str[i];
+        str[i] = str[n - i - 1];
+        str[n - i - 1] = temp;
+    }
+}
+
+int main() {
+    char str[100];
+    printf("Enter a string: ");
+    scanf("%s", str);
+
+    reverseString(str);
+
+    printf("Reversed string: %s\n", str);
+    return 0;
+}