diff --git a/labs/07_list_implementation/README.md b/labs/07_list_implementation/README.md
index 6a944a8..cb1c90f 100644
--- a/labs/07_list_implementation/README.md
+++ b/labs/07_list_implementation/README.md
@@ -36,7 +36,25 @@ Linked List of NodeB nodes: 1 -> 1.41421 -> 1.73205 -> 2 -> 2.23607 -> nullptr
 
 **To complete this checkpoint**, show a TA the implementation and the output of your program.
 
-## Checkpoint 3: Debugging a Merge Sort program. 
+## Checkpoint 3: Merge Two Lists. 
+*estimate: 30-40 minutes*
+
+Given two doubly-linked lists: linked list A and linked list B, and both linked lists are sorted. Data in linked list A is sorted in an ascending order. Data in linked list B is also sorted in an ascending order. Merge these two lists such that the data in the merged list is still sorted in an ascending order.
+
+More specifically, complete the mergeLists() function in [checkpoint3.cpp](checkpoint3.cpp), such that the program prints the following output.
+
+```console
+$ g++ checkpoint3.cpp
+$ ./a.out
+1 3 5 7 9
+2 4 6 8 10
+1 2 3 4 5 6 7 8 9 10
+10 9 8 7 6 5 4 3 2 1
+```
+
+**To complete this checkpoint**, explain to a TA your implementation and show the output of your program.
+
+<!--## Checkpoint 3: Debugging a Merge Sort program. 
 *estimate: 30-40 minutes*
 
 We expect our program [checkpoint3.cpp](checkpoint3.cpp) to produce the following results when it is compiled and run.
@@ -54,4 +72,4 @@ Sorted Vector: 2 3 4 7 8
 
 But this program currently does not behave as expected. Troubleshoot this program, find the problems and fix them. You can use a debugger.
 
-**To complete this checkpoint**, explain to a TA the bugs you found, show a TA your fixes and run the program to show that your fixes are correct and the program now produces the expected results.
+**To complete this checkpoint**, explain to a TA the bugs you found, show a TA your fixes and run the program to show that your fixes are correct and the program now produces the expected results.-->
diff --git a/labs/07_list_implementation/checkpoint3.cpp b/labs/07_list_implementation/checkpoint3.cpp
index 29177e4..2fc1e85 100644
--- a/labs/07_list_implementation/checkpoint3.cpp
+++ b/labs/07_list_implementation/checkpoint3.cpp
@@ -1,107 +1,93 @@
 #include <iostream>
-#include <vector>
 
-// prototype of the sorting function
-std::vector<int> sortVector(std::vector<int>& nums);
+template <class T>
+class Node {
+public:
+	T value;
+	Node<T>* next;
+	Node<T>* prev;
 
-// function to print the elements of a vector
-void printVector(const std::vector<int>& nums) {
-	for (int num : nums) {
-		std::cout << num << " ";
-	}
-	std::cout << std::endl;
+	// constructor
+	Node(T val) : value(val), next(nullptr), prev(nullptr) {}
+};
+
+// function to merge two sorted doubly linked lists
+template <class T>
+Node<T>* mergeLists(Node<T>* head_A, Node<T>* head_B) {
 }
 
 int main() {
-	// test case 1
-	std::vector<int> test1 = {5, 2, 9, 1, 5, 6};
-	std::vector<int> result1 = sortVector(test1);
-	std::cout << "Test Case 1: Original Vector: ";
-	printVector(test1);
-	std::cout << "Sorted Vector: ";
-	printVector(result1);
+	// create 5 nodes and link them to form a linked list, this is linked list A.
+	Node<int>* head_A = new Node<int>(1);
+	Node<int>* second_A = new Node<int>(3);
+	Node<int>* third_A = new Node<int>(5);
+	Node<int>* fourth_A = new Node<int>(7);
+	Node<int>* fifth_A = new Node<int>(9);
+
+	// link the nodes
+	head_A->next = second_A;
+	second_A->prev = head_A;
+	second_A->next = third_A;
+	third_A->prev = second_A;
+	third_A->next = fourth_A;
+	fourth_A->prev = third_A;
+	fourth_A->next = fifth_A;
+	fifth_A->prev = fourth_A;
+
+	// traverse linked list A and print the values
+	Node<int>* current = head_A;
+	while (current != nullptr) {
+		std::cout << current->value << " ";
+		current = current->next;
+	}
 	std::cout << std::endl;
 
-	// test case 2
-	std::vector<int> test2 = {3, 8, 2, 7, 4};
-	std::vector<int> result2 = sortVector(test2);
-	std::cout << "Test Case 2: Original Vector: ";
-	printVector(test2);
-	std::cout << "Sorted Vector: ";
-	printVector(result2);
+	// create 5 nodes and link them to form a linked list, this is linked list B.
+	Node<int>* head_B = new Node<int>(2);
+	Node<int>* second_B = new Node<int>(4);
+	Node<int>* third_B = new Node<int>(6);
+	Node<int>* fourth_B = new Node<int>(8);
+	Node<int>* fifth_B = new Node<int>(10);
+
+	// link the nodes
+	head_B->next = second_B;
+	second_B->prev = head_B;
+	second_B->next = third_B;
+	third_B->prev = second_B;
+	third_B->next = fourth_B;
+	fourth_B->prev = third_B;
+	fourth_B->next = fifth_B;
+	fifth_B->prev = fourth_B;
+
+	// traverse linked list B and print the values
+	current = head_B;
+	while (current != nullptr) {
+		std::cout << current->value << " ";
+		current = current->next;
+	}
+	std::cout << std::endl;
+
+	Node<int>* head_C;
+	Node<int>* tail_C;
+	head_C = mergeLists(head_A, head_B);
+
+	// traverse linked list C and print the values
+	current = head_C;
+	while (current != nullptr) {
+		std::cout << current->value << " ";
+		// keep tracking current and when current reaches nullptr, tail_C will be the tail node.
+		tail_C = current;
+		current = current->next;
+	}
+	std::cout << std::endl;
+
+	// traverse linked list C backwards and print the values
+	current = tail_C;
+	while (current != nullptr) {
+		std::cout << current->value << " ";
+		current = current->prev;
+	}
 	std::cout << std::endl;
 
 	return 0;
 }
-
-// merge two vectors which are already sorted
-std::vector<int>& mergeVectors(std::vector<int>& v1, std::vector<int>& v2){
-	int size1 = v1.size();
-	int size2 = v2.size();
-	std::vector<int> v(size1+size2, 0);
-	int index1 = 0;
-	int index2 = 0;
-	int index = 0;
-	// traverse v1 and v2 at the same time
-	while(index1<size1 && index2<size2){
-		// whoever is smaller goes to v
-		if(v1[index1]<v2[index2]){
-			v[index] = v1[index1];
-			index1 = index1 + 1;
-		}else{
-			v[index] = v2[index2];
-			index2 = index2 + 1;
-		}
-		index = index + 1;
-	}
-
-	// if v1 is done, let's now deal with v2 left overs
-	if(index1>=size1){
-		while(index2<size2){
-			v[index] = v2[index2];
-			index++;
-			index2++;
-		}
-	}else{
-		// else means v2 is done. let's now deal with v1 left overs
-		while(index1<size1){
-			v[index] = v1[index1];
-			index++;
-			index1++;
-		}
-	}
-	return v;
-}
-
-// sort a vecotr of integers
-std::vector<int> sortVector(std::vector<int>& nums) {
-	int size = nums.size();
-	// base case
-	if(size==1){
-		return nums;
-	}
-	// general case
-	// split the vector into two halves.
-	int mid = size/2;
-	
-	// nums1 to store the first half, and nums2 to store the second half.
-	std::vector<int> nums1;
-	std::vector<int> nums2;
-	
-	// copy the first half
-	for(int i=0;i<mid;i++){
-		nums1[i]=nums[i];
-	}
-	// copy the second half
-	for(int i=mid;i<size;i++){
-		nums2[i]=nums[i];
-	}
-
-	// make the recursive calls
-	nums1 = sortVector(nums1);
-	nums2 = sortVector(nums2);
-	
-	// now that the two vectors are already sorted, let's merge them
-	return mergeVectors(nums1, nums2);
-}
-