QuickSort on Singly Linked List

Difficulty Level : Hard
Last Updated : 28 May, 2021

QuickSort on Doubly Linked List is discussed here. QuickSort on Singly linked list was given as an exercise. Following is C++ implementation for same. The important things about implementation are, it changes pointers rather swapping data and time complexity is same as the implementation for Doubly Linked List.

sorting image

In partition(), we consider last element as pivot. We traverse through the current list and if a node has value greater than pivot, we move it after tail. If the node has smaller value, we keep it at its current position.

In QuickSortRecur(), we first call partition() which places pivot at correct position and returns pivot. After pivot is placed at correct position, we find tail node of left side (list before pivot) and recur for left list. Finally, we recur for right list.

C++

// C++ program for Quick Sort on Singly Linled List
#include <cstdio>
#include <iostream>
using namespace std;
 
/* a node of the singly linked list */
struct Node {
    int data;
    struct Node* next;
};
 
/* A utility function to insert a node at the beginning of
 * linked list */
void push(struct Node** head_ref, int new_data)
{
    /* allocate node */
    struct Node* new_node = new Node;
 
    /* put in the data */
    new_node->data = new_data;
 
    /* link the old list off the new node */
    new_node->next = (*head_ref);
 
    /* move the head to point to the new node */
    (*head_ref) = new_node;
}
 
/* A utility function to print linked list */
void printList(struct Node* node)
{
    while (node != NULL) {
        printf("%d ", node->data);
        node = node->next;
    }
    printf("\n");
}
 
// Returns the last node of the list
struct Node* getTail(struct Node* cur)
{
    while (cur != NULL && cur->next != NULL)
        cur = cur->next;
    return cur;
}
 
// Partitions the list taking the last element as the pivot
struct Node* partition(struct Node* head, struct Node* end,
                       struct Node** newHead,
                       struct Node** newEnd)
{
    struct Node* pivot = end;
    struct Node *prev = NULL, *cur = head, *tail = pivot;
 
    // During partition, both the head and end of the list
    // might change which is updated in the newHead and
    // newEnd variables
    while (cur != pivot) {
        if (cur->data < pivot->data) {
            // First node that has a value less than the
            // pivot - becomes the new head
            if ((*newHead) == NULL)
                (*newHead) = cur;
 
            prev = cur;
            cur = cur->next;
        }
        else // If cur node is greater than pivot
        {
            // Move cur node to next of tail, and change
            // tail
            if (prev)
                prev->next = cur->next;
            struct Node* tmp = cur->next;
            cur->next = NULL;
            tail->next = cur;
            tail = cur;
            cur = tmp;
        }
    }
 
    // If the pivot data is the smallest element in the
    // current list, pivot becomes the head
    if ((*newHead) == NULL)
        (*newHead) = pivot;
 
    // Update newEnd to the current last node
    (*newEnd) = tail;
 
    // Return the pivot node
    return pivot;
}
 
// here the sorting happens exclusive of the end node
struct Node* quickSortRecur(struct Node* head,
                            struct Node* end)
{
    // base condition
    if (!head || head == end)
        return head;
 
    Node *newHead = NULL, *newEnd = NULL;
 
    // Partition the list, newHead and newEnd will be
    // updated by the partition function
    struct Node* pivot
        = partition(head, end, &newHead, &newEnd);
 
    // If pivot is the smallest element - no need to recur
    // for the left part.
    if (newHead != pivot) {
        // Set the node before the pivot node as NULL
        struct Node* tmp = newHead;
        while (tmp->next != pivot)
            tmp = tmp->next;
        tmp->next = NULL;
 
        // Recur for the list before pivot
        newHead = quickSortRecur(newHead, tmp);
 
        // Change next of last node of the left half to
        // pivot
        tmp = getTail(newHead);
        tmp->next = pivot;
    }
 
    // Recur for the list after the pivot element
    pivot->next = quickSortRecur(pivot->next, newEnd);
 
    return newHead;
}
 
// The main function for quick sort. This is a wrapper over
// recursive function quickSortRecur()
void quickSort(struct Node** headRef)
{
    (*headRef)
        = quickSortRecur(*headRef, getTail(*headRef));
    return;
}
 
// Driver code
int main()
{
    struct Node* a = NULL;
    push(&a, 5);
    push(&a, 20);
    push(&a, 4);
    push(&a, 3);
    push(&a, 30);
 
    cout << "Linked List before sorting \n";
    printList(a);
 
    quickSort(&a);
 
    cout << "Linked List after sorting \n";
    printList(a);
 
    return 0;
}

Java

// Java program for Quick Sort on Singly Linled List
 
/*sort a linked list using quick sort*/
public
class QuickSortLinkedList {
    static class Node {
        int data;
        Node next;
 
        Node(int d)
        {
            this.data = d;
            this.next = null;
        }
    }
 
    Node head;
 
    void addNode(int data)
    {
        if (head == null) {
            head = new Node(data);
            return;
        }
 
        Node curr = head;
        while (curr.next != null)
            curr = curr.next;
 
        Node newNode = new Node(data);
        curr.next = newNode;
    }
 
    void printList(Node n)
    {
        while (n != null) {
            System.out.print(n.data);
            System.out.print(" ");
            n = n.next;
        }
    }
 
    // takes first and last node,
    // but do not break any links in
    // the whole linked list
    Node paritionLast(Node start, Node end)
    {
        if (start == end || start == null || end == null)
            return start;
 
        Node pivot_prev = start;
        Node curr = start;
        int pivot = end.data;
 
        // iterate till one before the end,
        // no need to iterate till the end
        // because end is pivot
        while (start != end) {
            if (start.data < pivot) {
                // keep tracks of last modified item
                pivot_prev = curr;
                int temp = curr.data;
                curr.data = start.data;
                start.data = temp;
                curr = curr.next;
            }
            start = start.next;
        }
 
        // swap the position of curr i.e.
        // next suitable index and pivot
        int temp = curr.data;
        curr.data = pivot;
        end.data = temp;
 
        // return one previous to current
        // because current is now pointing to pivot
        return pivot_prev;
    }
 
    void sort(Node start, Node end)
    {
        if(start == null || start == end|| start == end.next )
            return;
 
        // split list and partion recurse
        Node pivot_prev = paritionLast(start, end);
        sort(start, pivot_prev);
 
        // if pivot is picked and moved to the start,
        // that means start and pivot is same
        // so pick from next of pivot
        if (pivot_prev != null && pivot_prev == start)
            sort(pivot_prev.next, end);
 
        // if pivot is in between of the list,
        // start from next of pivot,
        // since we have pivot_prev, so we move two nodes
        else if (pivot_prev != null
                 && pivot_prev.next != null)
            sort(pivot_prev.next.next, end);
    }
 
    // Driver Code
public
    static void main(String[] args)
    {
        QuickSortLinkedList list
            = new QuickSortLinkedList();
        list.addNode(30);
        list.addNode(3);
        list.addNode(4);
        list.addNode(20);
        list.addNode(5);
 
        Node n = list.head;
        while (n.next != null)
            n = n.next;
 
        System.out.println("Linked List before sorting");
        list.printList(list.head);
 
        list.sort(list.head, n);
 
        System.out.println("\nLinked List after sorting");
        list.printList(list.head);
    }
}
 
// This code is contributed by trinadumca

Python3

'''
 
sort a linked list using quick sort
 
'''
 
class Node:
    def __init__(self, val):
        self.data = val
        self.next = None
 
class QuickSortLinkedList:
 
    def __init__(self):
        self.head=None
 
    def addNode(self,data):
        if (self.head == None):
            self.head = Node(data)
            return
 
        curr = self.head
        while (curr.next != None):
            curr = curr.next
 
        newNode = Node(data)
        curr.next = newNode
 
    def printList(self,n):
        while (n != None):
            print(n.data, end=" ")
            n = n.next
 
    ''' takes first and last node,but do not
    break any links in    the whole linked list'''
    def paritionLast(self,start, end):
        if (start == end or start == None or end == None):
            return start
 
        pivot_prev = start
        curr = start
        pivot = end.data
 
        '''iterate till one before the end,
        no need to iterate till the end because end is pivot'''
 
        while (start != end):
            if (start.data < pivot):
               
                # keep tracks of last modified item
                pivot_prev = curr
                temp = curr.data
                curr.data = start.data
                start.data = temp
                curr = curr.next
            start = start.next
 
        '''swap the position of curr i.e.
        next suitable index and pivot'''
 
        temp = curr.data
        curr.data = pivot
        end.data = temp
 
        ''' return one previous to current because
        current is now pointing to pivot '''
        return pivot_prev
 
    def sort(self, start, end):
        if(start == None or start == end or start == end.next):
            return
 
        # split list and partion recurse
        pivot_prev = self.paritionLast(start, end)
        self.sort(start, pivot_prev)
 
        '''
        if pivot is picked and moved to the start,
        that means start and pivot is same
        so pick from next of pivot
        '''
        if(pivot_prev != None and pivot_prev == start):
            self.sort(pivot_prev.next, end)
 
        # if pivot is in between of the list,start from next of pivot,
        # since we have pivot_prev, so we move two nodes
        elif (pivot_prev != None and pivot_prev.next != None):
            self.sort(pivot_prev.next.next, end)
 
if __name__ == "__main__":
    ll = QuickSortLinkedList()
    ll.addNode(30)
    ll.addNode(3)
    ll.addNode(4)
    ll.addNode(20)
    ll.addNode(5)
 
    n = ll.head
    while (n.next != None):
        n = n.next
 
    print("\nLinked List before sorting")
    ll.printList(ll.head)
 
    ll.sort(ll.head, n)
 
    print("\nLinked List after sorting");
    ll.printList(ll.head)
     
    # This code is contributed by humpheykibet.

C#

// C# program for Quick Sort on
// Singly Linled List
using System;
 
/*sort a linked list using quick sort*/
class GFG {
    public class Node {
        public int data;
        public Node next;
 
        public Node(int d)
        {
            this.data = d;
            this.next = null;
        }
    }
 
    Node head;
 
    void addNode(int data)
    {
        if (head == null) {
            head = new Node(data);
            return;
        }
 
        Node curr = head;
        while (curr.next != null)
            curr = curr.next;
 
        Node newNode = new Node(data);
        curr.next = newNode;
    }
 
    void printList(Node n)
    {
        while (n != null) {
            Console.Write(n.data);
            Console.Write(" ");
            n = n.next;
        }
    }
 
    // takes first and last node,
    // but do not break any links in
    // the whole linked list
    Node paritionLast(Node start, Node end)
    {
        if (start == end || start == null || end == null)
            return start;
 
        Node pivot_prev = start;
        Node curr = start;
        int pivot = end.data;
 
        // iterate till one before the end,
        // no need to iterate till the end
        // because end is pivot
        int temp;
        while (start != end) {
 
            if (start.data < pivot) {
                // keep tracks of last modified item
                pivot_prev = curr;
                temp = curr.data;
                curr.data = start.data;
                start.data = temp;
                curr = curr.next;
            }
            start = start.next;
        }
 
        // swap the position of curr i.e.
        // next suitable index and pivot
        temp = curr.data;
        curr.data = pivot;
        end.data = temp;
 
        // return one previous to current
        // because current is now pointing to pivot
        return pivot_prev;
    }
 
    void sort(Node start, Node end)
    {
        if (start == end)
            return;
 
        // split list and partion recurse
        Node pivot_prev = paritionLast(start, end);
        sort(start, pivot_prev);
 
        // if pivot is picked and moved to the start,
        // that means start and pivot is same
        // so pick from next of pivot
        if (pivot_prev != null && pivot_prev == start)
            sort(pivot_prev.next, end);
 
        // if pivot is in between of the list,
        // start from next of pivot,
        // since we have pivot_prev, so we move two nodes
        else if (pivot_prev != null
                 && pivot_prev.next != null)
            sort(pivot_prev.next.next, end);
    }
 
    // Driver Code
    public static void Main(String[] args)
    {
        GFG list = new GFG();
        list.addNode(30);
        list.addNode(3);
        list.addNode(4);
        list.addNode(20);
        list.addNode(5);
 
        Node n = list.head;
        while (n.next != null)
            n = n.next;
 
        Console.WriteLine("Linked List before sorting");
        list.printList(list.head);
 
        list.sort(list.head, n);
 
        Console.WriteLine("\nLinked List after sorting");
        list.printList(list.head);
    }
}
 
// This code is contributed by 29AjayKumar

Javascript

<script>
// javascript program for Quick Sort on Singly Linled List
 
/*sort a linked list using quick sort*/
 
     class Node {
        constructor(val) {
            this.data = val;
            this.next = null;
        }
    }
 
    var head;
 
    function addNode(data) {
        if (head == null) {
            head = new Node(data);
            return;
        }
 
        var curr = head;
        while (curr.next != null)
            curr = curr.next;
 
        var newNode = new Node(data);
        curr.next = newNode;
    }
 
    function printList( n) {
        while (n != null) {
            document.write(n.data);
            document.write(" ");
            n = n.next;
        }
    }
 
    // takes first and last node,
    // but do not break any links in
    // the whole linked list
    function paritionLast( start,  end) {
        if (start == end || start == null || end == null)
            return start;
 
        var pivot_prev = start;
        var curr = start;
        var pivot = end.data;
 
        // iterate till one before the end,
        // no need to iterate till the end
        // because end is pivot
        while (start != end) {
            if (start.data < pivot) {
                // keep tracks of last modified item
                pivot_prev = curr;
                var temp = curr.data;
                curr.data = start.data;
                start.data = temp;
                curr = curr.next;
            }
            start = start.next;
        }
 
        // swap the position of curr i.e.
        // next suitable index and pivot
        var temp = curr.data;
        curr.data = pivot;
        end.data = temp;
 
        // return one previous to current
        // because current is now pointing to pivot
        return pivot_prev;
    }
 
    function sort( start,  end) {
        if (start == null || start == end || start == end.next)
            return;
 
        // split list and partion recurse
        var pivot_prev = paritionLast(start, end);
        sort(start, pivot_prev);
 
        // if pivot is picked and moved to the start,
        // that means start and pivot is same
        // so pick from next of pivot
        if (pivot_prev != null && pivot_prev == start)
            sort(pivot_prev.next, end);
 
        // if pivot is in between of the list,
        // start from next of pivot,
        // since we have pivot_prev, so we move two nodes
        else if (pivot_prev != null && pivot_prev.next != null)
            sort(pivot_prev.next.next, end);
    }
 
    // Driver Code
     
        addNode(30);
        addNode(3);
        addNode(4);
        addNode(20);
        addNode(5);
 
        var n = head;
        while (n.next != null)
            n = n.next;
 
        document.write("Linked List before sorting<br/>");
        printList(head);
 
        sort(head, n);
 
        document.write("<br/>Linked List after sorting<br/>");
        printList(head);
 
// This code contributed by umadevi9616
</script>

Output

Linked List before sorting 
30 3 4 20 5 
Linked List after sorting 
3 4 5 20 30

This article is contributed by Balasubramanian.N . Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above

Attention reader! Don’t stop learning now. Get hold of all the important DSA concepts with the DSA Self Paced Course at a student-friendly price and become industry ready. To complete your preparation from learning a language to DS Algo and many more, please refer Complete Interview Preparation Course.

In case you wish to attend live classes with industry experts, please refer DSA Live Classes

My Personal Notes

Related Articles

C++

Java

Python3

C#

Javascript