Java Program For QuickSort On Doubly Linked List
Last Updated :
03 May, 2023
Following is a typical recursive implementation of QuickSort for arrays. The implementation uses last element as pivot.
Java
static int partition ( int []arr, int l, int h)
{
int x = arr[h];
int i = (l - 1 );
for ( int j = l; j <= h - 1 ; j++)
{
if (arr[j] <= x)
{
i++;
int tmp = arr[i];
arr[i] = arr[j];
arr[j] = tmp;
}
}
int tmp = arr[i + 1 ];
arr[i + 1 ] = arr[h];
arr[h] = tmp;
return (i + 1 );
}
static void quickSort( int []A, int l,
int h)
{
if (l < h)
{
int p = partition(A, l, h);
quickSort(A, l, p - 1 );
quickSort(A, p + 1 , h);
}
}
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Can we use the same algorithm for Linked List?
Following is C++ implementation for the doubly linked list. The idea is simple, we first find out pointer to the last node. Once we have a pointer to the last node, we can recursively sort the linked list using pointers to first and last nodes of a linked list, similar to the above recursive function where we pass indexes of first and last array elements. The partition function for a linked list is also similar to partition for arrays. Instead of returning index of the pivot element, it returns a pointer to the pivot element. In the following implementation, quickSort() is just a wrapper function, the main recursive function is _quickSort() which is similar to quickSort() for array implementation.
Java
class QuickSort_using_Doubly_LinkedList{
Node head;
static class Node{
private int data;
private Node next;
private Node prev;
Node( int d){
data = d;
next = null ;
prev = null ;
}
}
Node lastNode(Node node){
while (node.next!= null )
node = node.next;
return node;
}
Node partition(Node l,Node h)
{
int x = h.data;
Node i = l.prev;
for (Node j=l; j!=h; j=j.next)
{
if (j.data <= x)
{
i = (i== null ) ? l : i.next;
int temp = i.data;
i.data = j.data;
j.data = temp;
}
}
i = (i== null ) ? l : i.next;
int temp = i.data;
i.data = h.data;
h.data = temp;
return i;
}
void _quickSort(Node l,Node h)
{
if (h!= null && l!=h && l!=h.next){
Node temp = partition(l,h);
_quickSort(l,temp.prev);
_quickSort(temp.next,h);
}
}
public void quickSort(Node node)
{
Node head = lastNode(node);
_quickSort(node,head);
}
public void printList(Node head)
{
while (head!= null ){
System.out.print(head.data+ " " );
head = head.next;
}
}
void push( int new_Data)
{
Node new_Node = new Node(new_Data);
if (head== null ){
head = new_Node;
return ;
}
new_Node.next = head;
head.prev = new_Node;
new_Node.prev = null ;
head = new_Node;
}
public static void main(String[] args){
QuickSort_using_Doubly_LinkedList list = new QuickSort_using_Doubly_LinkedList();
list.push( 5 );
list.push( 20 );
list.push( 4 );
list.push( 3 );
list.push( 30 );
System.out.println( "Linked List before sorting " );
list.printList(list.head);
System.out.println("
Linked List after sorting");
list.quickSort(list.head);
list.printList(list.head);
}
}
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Output :
Linked List before sorting
30 3 4 20 5
Linked List after sorting
3 4 5 20 30
Time Complexity: Time complexity of the above implementation is same as time complexity of QuickSort() for arrays. It takes O(n^2) time in the worst case and O(nLogn) in average and best cases. The worst case occurs when the linked list is already sorted.
Space Complexity: O(n). The extra space is due to the function call stack.
Can we implement random quicksort for a linked list?
Quicksort can be implemented for Linked List only when we can pick a fixed point as the pivot (like the last element in the above implementation). Random QuickSort cannot be efficiently implemented for Linked Lists by picking random pivot.
Please refer complete article on QuickSort on Doubly Linked List for more details!
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