C Exercises: Combine k sorted linked lists into a single sorted linked list

Last update on August 04 2025 13:01:35 (UTC/GMT +8 hours)

29. Merging K Sorted Lists Challenges

Write a C program to merge k sorted linked lists into a single sorted linked list.

Sample Solution:

C Code:

#include<stdio.h>
#include <stdlib.h>

// Structure for defining a Node in a Singly Linked List
struct Node {
    int data; // Data stored in the node
    struct Node* next; // Pointer to the next node
};

// Function to create a new node in the Singly Linked List
struct Node* newNode(int data) {
    struct Node* temp = (struct Node*) malloc(sizeof(struct Node)); // Allocate memory for a new node
    temp->data = data; // Assign the data to the new node
    temp->next = NULL; // Initialize next pointer as NULL
    return temp; // Return the new node
}

// Function to display a linked list
void displayList(struct Node* head) {
    while (head) {
        printf("%d ", head->data); // Print the data of the current node
        head = head->next; // Move to the next node
    }
    printf("\n");
}

// Function to merge two sorted linked singly_lists into a single sorted linked list
struct Node* mergeTwoLists(struct Node* l1, struct Node* l2) {
    if (!l1) return l2; // If l1 is empty, return l2
    if (!l2) return l1; // If l2 is empty, return l1

    if (l1->data < l2->data) {
        l1->next = mergeTwoLists(l1->next, l2); // Merge remaining lists after l1
        return l1; // Return merged list with l1 as the starting node
    } else {
        l2->next = mergeTwoLists(l1, l2->next); // Merge remaining lists after l2
        return l2; // Return merged list with l2 as the starting node
    }
}

// Function to merge k sorted lists into one sorted list
struct Node* mergeKLists(struct Node** lists, int listsSize) {
    if (listsSize == 0) return NULL; // If there are no lists, return NULL
    if (listsSize == 1) return lists[0]; // If only one list, return that list
    if (listsSize == 2) return mergeTwoLists(lists[0], lists[1]); // If two lists, merge them

    int mid = listsSize / 2;
    struct Node *left = mergeKLists(lists, mid); // Merge the first half of lists
    struct Node *right = mergeKLists(lists + mid, listsSize - mid); // Merge the second half of lists

    return mergeTwoLists(left, right); // Merge the two merged halves
}

// Main function
int main() {
    int k = 3; // Number of lists to be merged
    struct Node* singly_lists[3]; // Array of pointers to linked lists

    singly_lists[0] = newNode(10); // Create and populate List-1
    singly_lists[0]->next = newNode(20);
    singly_lists[0]->next->next = newNode(50);
    printf("List-1:\n");
    displayList(singly_lists[0]);

    singly_lists[1] = newNode(30); // Create and populate List-2
    singly_lists[1]->next = newNode(40);
    singly_lists[1]->next->next = newNode(60);
    printf("List-2:\n");
    displayList(singly_lists[1]);

    singly_lists[2] = newNode(10); // Create and populate List-3
    singly_lists[2]->next = newNode(70);
    singly_lists[2]->next->next = newNode(100);
    printf("List-3:\n");
    displayList(singly_lists[2]);

    struct Node* result = mergeKLists(singly_lists, k); // Merge the k lists
    printf("\nAfter merging the said three sorted lists into one sorted list:\n");
    displayList(result); // Display the merged list

    return 0;
}

Sample Output:

List-1:
10 20 50 
List-2:
30 40 60 
List-3:
10 70 100 

After merging the said three sorted lists into one sorted list:
10 10 20 30 40 50 60 70 100

Flowchart :

For more Practice: Solve these Related Problems:

Write a C program to merge k sorted linked lists using a divide and conquer strategy.
Write a C program to merge k sorted linked lists and simultaneously remove duplicate nodes during the merge.
Write a C program to merge k sorted linked lists where each list is sorted in descending order, then output in ascending order.
Write a C program to merge k sorted linked lists using a simulated min-heap approach in C.

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