add solution of lab 5

labs/list_implementation/checkpoint3.cpp

@@ -15,6 +15,51 @@ public:
 // this function returns a pointer pointing to the head node of the merged list.
 template <class T>
 Node<T>* mergeLists(Node<T>* head_A, Node<T>* head_B) {
+    // If one list is empty, return the other
+    if (!head_A) return head_B;
+    if (!head_B) return head_A;
+
+    // Decide the new head by comparing the first elements
+    Node<T>* head = nullptr;
+    Node<T>* tail = nullptr;
+
+    // Pick whichever head is smaller as the new head
+    if (head_A->value < head_B->value) {
+        head = head_A;
+        head_A = head_A->next;
+    } else {
+        head = head_B;
+        head_B = head_B->next;
+    }
+    // At this point, 'head' is the first node of the merged list
+    head->prev = nullptr; // new head has no previous
+    tail = head;
+
+    // Continue merging while both lists have nodes
+    while (head_A && head_B) {
+        if (head_A->value < head_B->value) {
+            tail->next = head_A;
+            head_A->prev = tail;
+            tail = tail->next;
+            head_A = head_A->next;
+        } else {
+            tail->next = head_B;
+            head_B->prev = tail;
+            tail = tail->next;
+            head_B = head_B->next;
+        }
+    }
+
+    // Append any remaining nodes from A or B
+    if (head_A) {
+        tail->next = head_A;
+        head_A->prev = tail;
+    } else if (head_B) {
+        tail->next = head_B;
+        head_B->prev = tail;
+    }
+
+    return head;
 }
 
 int main() {