Sort a Linked List Using Merge Sort in DSA C - Build from Scratch

Sort a Linked List Using Merge Sort

📖 Scenario: You are working on a program that manages a list of tasks. Each task has a priority number. To organize tasks efficiently, you want to sort the tasks by their priority using a linked list.

🎯 Goal: Build a program that creates a linked list of tasks with priorities, then sorts this linked list using the merge sort algorithm.

📋 What You'll Learn

Create a linked list with given task priorities

Implement a function to split the linked list into halves

Implement a function to merge two sorted linked lists

Implement merge sort to sort the linked list

Print the sorted linked list priorities

💡 Why This Matters

🌍 Real World

Sorting linked lists is useful in many applications like task scheduling, managing priority queues, and organizing data streams where dynamic memory allocation is preferred.

💼 Career

Understanding linked list sorting algorithms like merge sort is important for software engineering roles that involve data structure optimization and memory-efficient programming.

1

Create the Linked List

Create a linked list with nodes containing these exact priority values in order: 4, 2, 5, 1, 3. Use the struct Node with an int priority and a Node* next. Create a pointer head pointing to the first node.

DSA C

#include 
#include 

struct Node {
    int priority;
    struct Node* next;
};

int main() {
    // Create linked list nodes with priorities 4, 2, 5, 1, 3
    // Your code here
    return 0;
}

Hint

Use malloc to create each node. Link them by setting the next pointer of each node to the next node.

2

Add a Function to Split the List

Add a function called splitList that takes a struct Node* called source and splits it into two halves. It should set two pointers frontRef and backRef to the front and back halves respectively. Use the fast and slow pointer method.

DSA C

#include 
#include 

struct Node {
    int priority;
    struct Node* next;
};

// Add the splitList function here
// Your code here

int main() {
    struct Node* head = malloc(sizeof(struct Node));
    head->priority = 4;
    head->next = malloc(sizeof(struct Node));
    head->next->priority = 2;
    head->next->next = malloc(sizeof(struct Node));
    head->next->next->priority = 5;
    head->next->next->next = malloc(sizeof(struct Node));
    head->next->next->next->priority = 1;
    head->next->next->next->next = malloc(sizeof(struct Node));
    head->next->next->next->next->priority = 3;
    head->next->next->next->next->next = NULL;
    return 0;
}

Hint

Use two pointers: slow moves one step, fast moves two steps. When fast reaches the end, slow is at the middle.

3

Add Merge and Merge Sort Functions

Add two functions: mergeSortedLists that merges two sorted linked lists into one sorted list, and mergeSort that sorts a linked list using merge sort recursively. Use the splitList function to divide the list.

DSA C

#include 
#include 

struct Node {
    int priority;
    struct Node* next;
};

void splitList(struct Node* source, struct Node** frontRef, struct Node** backRef) {
    struct Node* slow;
    struct Node* fast;
    if (source == NULL || source->next == NULL) {
        *frontRef = source;
        *backRef = NULL;
        return;
    }
    slow = source;
    fast = source->next;
    while (fast != NULL) {
        fast = fast->next;
        if (fast != NULL) {
            slow = slow->next;
            fast = fast->next;
        }
    }
    *frontRef = source;
    *backRef = slow->next;
    slow->next = NULL;
}

// Add mergeSortedLists and mergeSort functions here
// Your code here

int main() {
    struct Node* head = malloc(sizeof(struct Node));
    head->priority = 4;
    head->next = malloc(sizeof(struct Node));
    head->next->priority = 2;
    head->next->next = malloc(sizeof(struct Node));
    head->next->next->priority = 5;
    head->next->next->next = malloc(sizeof(struct Node));
    head->next->next->next->priority = 1;
    head->next->next->next->next = malloc(sizeof(struct Node));
    head->next->next->next->next->priority = 3;
    head->next->next->next->next->next = NULL;
    return 0;
}

Hint

Use recursion in mergeSort. Base case is when list is empty or has one node. Use mergeSortedLists to combine sorted halves.

4

Print the Sorted Linked List

Add a function called printList that takes struct Node* head and prints the priorities in order separated by spaces. Then call printList(head) in main after sorting.

DSA C

Hint

Traverse the list from head to NULL, printing each priority followed by a space.