Sorting given Character Array using Linked List

In this problem, we need to sort the given array of characters using a linked list. We can use bubble sort, selection sort, merger sort, etc. techniques to sort the array.

Here, we will convert the array into the linked list first and then use the selection sort and bubble sort techniques to sort the array.

Problem statement − We have given an array arr[] of length N. The array contains lowercase alphabetical characters. We need to sort the array using the linked list.

Sample examples

Input

arr[] = {'e', 's', 'a', 'x', 'c', 'e','f', 'p', 'b', 'n', 'a'};

Output

a -> a -> b -> c -> e -> e -> f -> n -> p -> s -> x -> NULL

Explanation − We have sorted the array in ascending order.

Input

 arr[] = {'g', 'g', 'h', 'i', 'j', 'k', 'k', 'l', 'm', 'n', 'o'};

Output

g -> g -> h -> i -> j -> k -> k -> l -> m -> n -> o -> NULL

Explanation − The array arr[] and output are the same as the array is already sorted.

Approach 1

In this approach, we will use the bubble sort algorithm to sort the linked list. First, we will convert the array to a linked list and then use the bubble sort technique. In the bubble sort technique, we make total N iterations for swapping all list elements that are greater than the next element.

Algorithm

Step 1 − The sortLinkedList() function is used to sort the linked list, which takes the start node and length of the list as a parameter.

Step 2 − Define the ‘curr’ node and ‘swapped’ boolean variable to keep track in single iterations any swapping occurred or not.

Step 3 − Use the loop to traverse the linked list. Assign the ‘start’ node to ‘curr’ and initialize the ‘swapped’ with false.

Step 4 − Now, use another nested loop to make iterations until len − p −1. Define ‘temp1’ and ‘temp2’ variables and initialize them with the ‘curr’ node and its next node, respectively.

Step 5 − If temp1 −> ch is greater than temp2 −> ch, swap them and update the value of the ‘curr’ node and ‘swapped’ variable.

Step 6 − Move ‘curr’ to the next node.

Step 7 − If ‘swapped == false’ and not updated while making the iterations using a nested loop, break the outer loop as the list is already sorted.

Example

In this example, we used the structure to create a linked list. Also, we defined the addNode() function to insert the node into the current list. We take each element of the array and use the addNode() function to add a character to the list.

The addNode() function traverses the list until it gets the last node and adds the new node at last.

#include <iostream>
using namespace std;

// creating the struct listNode
struct listNode {
    int ch;
    listNode *next;
} listNode;

// Swapping the nodes of the linked list
struct listNode *swap(struct listNode *pointer1, struct listNode *pointer2) {
    struct listNode *tmp = pointer2->next;
    pointer2->next = pointer1;
    pointer1->next = tmp;
    return pointer2;
}
// Sorting the linked list
void sortLinkedList(struct listNode **start, int len) {
    // storing the current node
    struct listNode **curr;
    int p, q;
    bool swapped = false;
    for (p = 0; p <= len; p++) {
        // store the current node address in curr
        curr = start;
        swapped = false;
        // traverse the list
        for (q = 0; q < len - p - 1; q++) {
            struct listNode *temp1 = *curr;
            struct listNode *temp2 = temp1->next;
            // If temp1 > temp2 swap them
            if (temp1->ch > temp2->ch) {
                // Update the link after swapping
                *curr = swap(temp1, temp2);
                swapped = true;
            }
            // Move the current pointer to the next node
            curr = &(*curr)->next;
        }
        // If any pair of elements is not swapped, break the loop.
        if (swapped == false)
            break;
    }
}
// adding nodes to the linked list
void addNode(struct listNode **start, int ch) {
    // creating a new node
    struct listNode *temp = new struct listNode();
    // add ch to the node
    temp->ch = ch;
    temp->next = NULL;
    // If the list is empty, add a node to the list
    if (*start == NULL) {
        *start = temp;
    } else {
        // If the list has some nodes, append the node at the end of the list
        struct listNode *pointer1 = *start;
        while (pointer1->next != NULL) {
            pointer1 = pointer1->next;
        }
        pointer1->next = temp;
    }
}
// print nodes
void printLinkedList(struct listNode *head) {
    cout << "The sorted list is " << endl;
    while (head != NULL) {
        cout << char(head->ch) << " -> ";
        head = head->next;
    }
    cout << "NULL" << endl;
}
int main() {
    int arr[] = {'e', 's', 'a', 'x', 'c', 'e', 'f', 'p', 'b', 'n', 'a'};
    int len, p;
    // create an empty linked list
    struct listNode *start = NULL;
    len = sizeof(arr) / sizeof(arr[0]);
    // inserting characters of the array to a linked list
    for (p = 0; p < len; p++)
        addNode(&start, arr[p]);
    // Sort the list
    sortLinkedList(&start, len);
    printLinkedList(start);
    return 0;
}

Output

The sorted list is 
a -> a -> b -> c -> e -> e -> f -> n -> p -> s -> x -> NULL

Time complexity− O(N^2) as we use two nested loops.

Space complexity − O(1) as we don’t use dynamic space in the sortLinkedList() function. However, we use O(N) space when we convert an array to a list, but it is not a part of sorting.

Approach 2

We will use the selection sort algorithm to sort the linked list in this approach. In the selection sort algorithm, we find the node containing the minimum value from the suffix of len − p. After that, we swap the minimum node with the node at the pth index.

Algorithm

Step 1 − Define and initialize the ‘current’ node with the ‘start’ node.

Step 2 − Use the while loop to make iterations until the ‘current’ node is not null.

Step 3 − Initialize the ‘minNode’ with the ‘current’ node and ‘traverse’ with the next current, as we need to find the minimum node.

Step 4 − Use the nested while loop to find a minimum node and swap it with the ‘current’ node.

Step 5 − Move the current node to the next.

Example

#include <iostream>
using namespace std;

struct listNode {
    int ch;
    listNode* next;
};

void swapNodes(struct listNode* node1, struct listNode* node2) {
    int temp = node1->ch;
    node1->ch = node2->ch;
    node2->ch = temp;
}

void sortLinkedList(struct listNode* start) {
    struct listNode* current = start;
    while (current != nullptr) {
        struct listNode* minNode = current;
        struct listNode* traverse = current->next;
        while (traverse != nullptr) {
            if (traverse->ch < minNode->ch) {
                minNode = traverse;
            }
            traverse = traverse->next;
        }
        swapNodes(current, minNode);
        current = current->next;
    }
}

void addNode(struct listNode** start, int ch) {
    struct listNode* newNode = new struct listNode();
    newNode->ch = ch;
    newNode->next = nullptr;
    if (*start == nullptr) {
        *start = newNode;
    } else {
        struct listNode* current = *start;
        while (current->next != nullptr) {
            current = current->next;
        }
        current->next = newNode;
    }
}
void printLinkedList(struct listNode* head) {
    cout << "The sorted list is " << endl;
    while (head != nullptr) {
        cout << char(head->ch) << " -> ";
        head = head->next;
    }
    cout << "NULL" << endl;
}

int main() {
    int arr[] = {'e', 's', 'a', 'x', 'c', 'e', 'f', 'p', 'b', 'n', 'a'};
    int len = sizeof(arr) / sizeof(arr[0]);
    struct listNode* start = nullptr;
    for (int p = 0; p < len; p++) {
        addNode(&start, arr[p]);
    }
    sortLinkedList(start);
    printLinkedList(start);
    return 0;
}

Output

The sorted list is 
a -> a -> b -> c -> e -> e -> f -> n -> p -> s -> x -> NULL

Time complexity− O(N^2) as we use two nested while loops.

Space complexity − O(1) as we don’t use dynamic space.