Java Program For Sorting A Linked List Of 0s, 1s And 2s

Given a linked list of 0s, 1s and 2s, sort it.
Examples:

Input: 1 -> 1 -> 2 -> 0 -> 2 -> 0 -> 1 -> NULL
Output: 0 -> 0 -> 1 -> 1 -> 1 -> 2 -> 2 -> NULL

Input: 1 -> 1 -> 2 -> 1 -> 0 -> NULL 
Output: 0 -> 1 -> 1 -> 1 -> 2 -> NULL

Source: Microsoft Interview | Set 1

Following steps can be used to sort the given linked list.

• Traverse the list and count the number of 0s, 1s, and 2s. Let the counts be n1, n2, and n3 respectively.
• Traverse the list again, fill the first n1 nodes with 0, then n2 nodes with 1, and finally n3 nodes with 2.

Below image is a dry run of the above approach:

Below is the implementation of the above approach:

Java

// Java program to sort a linked list
// of 0, 1 and 2
class LinkedList
{
    // Head of list
    Node head;

    // Linked list Node
    class Node
    {
        int data;
        Node next;
        Node(int d)
        {
            data = d;
            next = null;
        }
    }

    void sortList()
    {
       // Initialise count of 0 1
       // and 2 as 0
       int count[] = {0, 0, 0};

       Node ptr = head;

       /* Count total number of '0', '1' and '2'
          count[0] will store total number of '0's
          count[1] will store total number of '1's
          count[2] will store total number of '2's  */
       while (ptr != null)
       {
            count[ptr.data]++;
            ptr = ptr.next;
       }

       int i = 0;
       ptr = head;

       /* Let say count[0] = n1, count[1] = n2
          and count[2] = n3
          now start traversing list from head node,
          1) fill the list with 0, till n1 > 0
          2) fill the list with 1, till n2 > 0
          3) fill the list with 2, till n3 > 0  */
        while (ptr != null)
        {
            if (count[i] == 0)
                i++;
            else
            {
               ptr.data= i;
               --count[i];
               ptr = ptr.next;
            }
         }
    }

    // Utility functions
    /* Inserts a new Node at front
       of the list. */
    public void push(int new_data)
    {
        /* 1 & 2: Allocate the Node &
                  Put in the data*/
        Node new_node = new Node(new_data);

        // 3. Make next of new Node as head
        new_node.next = head;

        // 4. Move the head to point to
        // new Node
        head = new_node;
    }

    // Function to print linked list
    void printList()
    {
        Node temp = head;
        while (temp != null)
        {
           System.out.print(temp.data + " ");
           temp = temp.next;
        }
        System.out.println();
    }

    // Driver code
    public static void main(String args[])
    {
        LinkedList llist = new LinkedList();

        /* Constructed Linked List is
           1->2->3->4->5->6->7->8->8->9->null */
        llist.push(0);
        llist.push(1);
        llist.push(0);
        llist.push(2);
        llist.push(1);
        llist.push(1);
        llist.push(2);
        llist.push(1);
        llist.push(2);

        System.out.println(
               "Linked List before sorting");
        llist.printList();

        llist.sortList();

        System.out.println(
               "Linked List after sorting");
        llist.printList();
    }
}
// This code is contributed by Rajat Mishra

Output:

Linked List Before Sorting
2  1  2  1  1  2  0  1  0
Linked List After Sorting
0  0  1  1  1  1  2  2  2

Time Complexity: O(n) where n is the number of nodes in the linked list. 
Auxiliary Space: O(1)

Please refer complete article on Sort a linked list of 0s, 1s and 2s for more details!