Java Program For Moving All Occurrences Of An Element To End In A Linked List

Given a linked list and a key in it, the task is to move all occurrences of the given key to the end of the linked list, keeping the order of all other elements the same.

Examples:  

Input  : 1 -> 2 -> 2 -> 4 -> 3
         key = 2 
Output : 1 -> 4 -> 3 -> 2 -> 2

Input  : 6 -> 6 -> 7 -> 6 -> 3 -> 10
         key = 6
Output : 7 -> 3 -> 10 -> 6 -> 6 -> 6

A simple solution is to one by one find all occurrences of a given key in the linked list. For every found occurrence, insert it at the end. We do it till all occurrences of the given key are moved to the end.

Time Complexity: O(n²)

Efficient Solution 1: is to keep two pointers: 
pCrawl => Pointer to traverse the whole list one by one. 
pKey => Pointer to an occurrence of the key if a key is found. Else same as pCrawl.
We start both of the above pointers from the head of the linked list. We move pKey only when pKey is not pointing to a key. We always move pCrawl. So, when pCrawl and pKey are not the same, we must have found a key that lies before pCrawl, so we swap between pCrawl and pKey, and move pKey to the next location. The loop invariant is, after swapping of data, all elements from pKey to pCrawl are keys.

Below is the implementation of this approach.  

Java

// Java program to move all occurrences of a
// given key to end.
class GFG {

    // A Linked list Node
    static class Node {
        int data;
        Node next;
    }

    // A utility function to create a new node.
    static Node newNode(int x)
    {
        Node temp = new Node();
        temp.data = x;
        temp.next = null;
        return temp;
    }

    // Utility function to print the elements
    // in Linked list
    static void printList(Node head)
    {
        Node temp = head;
        while (temp != null) {
            System.out.printf("%d ", temp.data);
            temp = temp.next;
        }
        System.out.printf("
");
    }

    // Moves all occurrences of given key to
    // end of linked list.
    static void moveToEnd(Node head, int key)
    {
        // Keeps track of locations where key
        // is present.
        Node pKey = head;

        // Traverse list
        Node pCrawl = head;
        while (pCrawl != null) {
            // If current pointer is not same as pointer
            // to a key location, then we must have found
            // a key in linked list. We swap data of pCrawl
            // and pKey and move pKey to next position.
            if (pCrawl != pKey && pCrawl.data != key) {
                pKey.data = pCrawl.data;
                pCrawl.data = key;
                pKey = pKey.next;
            }

            // Find next position where key is present
            if (pKey.data != key)
                pKey = pKey.next;

            // Moving to next Node
            pCrawl = pCrawl.next;
        }
    }

    // Driver code
    public static void main(String args[])
    {
        Node head = newNode(10);
        head.next = newNode(20);
        head.next.next = newNode(10);
        head.next.next.next = newNode(30);
        head.next.next.next.next = newNode(40);
        head.next.next.next.next.next = newNode(10);
        head.next.next.next.next.next.next = newNode(60);

        System.out.printf("Before moveToEnd(), the Linked list is
");
        printList(head);

        int key = 10;
        moveToEnd(head, key);

        System.out.printf("
After moveToEnd(), the Linked list is
");
        printList(head);
    }
}

// This code is contributed by Arnab Kundu

Output:

Before moveToEnd(), the Linked list is
10 20 10 30 40 10 60 

After moveToEnd(), the Linked list is
20 30 40 60 10 10 10

Time Complexity: O(n) requires only one traversal of the list.

Auxiliary Space: O(1)

Efficient Solution 2 :
1. Traverse the linked list and take a pointer at the tail. 
2. Now, check for the key and node->data. If they are equal, move the node to last-next, else move ahead.

Java

// Java code to remove key element to end of linked list
import java.util.*;

// Node class
class Node {
    int data;
    Node next;

    public Node(int data)
    {
        this.data = data;
        this.next = null;
    }
}

class gfg {

    static Node root;

    // Function to remove key to end
    public static Node keyToEnd(Node head, int key)
    {

        // Node to keep pointing to tail
        Node tail = head;

        if (head == null) {
            return null;
        }

        while (tail.next != null) {
            tail = tail.next;
        }

        // Node to point to last of linked list
        Node last = tail;

        Node current = head;
        Node prev = null;

        // Node prev2 to point to previous when head.data!=key
        Node prev2 = null;

        // loop to perform operations to remove key to end
        while (current != tail) {
            if (current.data == key && prev2 == null) {
                prev = current;
                current = current.next;
                head = current;
                last.next = prev;
                last = last.next;
                last.next = null;
                prev = null;
            }
            else {
                if (current.data == key && prev2 != null) {
                    prev = current;
                    current = current.next;
                    prev2.next = current;
                    last.next = prev;
                    last = last.next;
                    last.next = null;
                }
                else if (current != tail) {
                    prev2 = current;
                    current = current.next;
                }
            }
        }
        return head;
    }

    // Function to display linked list
    public static void display(Node root)
    {
        while (root != null) {
            System.out.print(root.data + " ");
            root = root.next;
        }
    }

    // Driver Code
    public static void main(String args[])
    {
        root = new Node(5);
        root.next = new Node(2);
        root.next.next = new Node(2);
        root.next.next.next = new Node(7);
        root.next.next.next.next = new Node(2);
        root.next.next.next.next.next = new Node(2);
        root.next.next.next.next.next.next = new Node(2);

        int key = 2;
        System.out.println("Linked List before operations :");
        display(root);
        System.out.println("
Linked List after operations :");
        root = keyToEnd(root, key);
        display(root);
    }
}

Output:

Linked List before operations :
5 2 2 7 2 2 2 
Linked List after operations :
5 7 2 2 2 2 2

Time Complexity: O(n), where n represents the size of the given list.
Auxiliary Space: O(1), no extra space is required, so it is a constant.

Thanks to Ravinder Kumar for suggesting this method.

Efficient Solution 3: is to maintain a separate list of keys. We initialize this list of keys as empty. We traverse the given list. For every key found, we remove it from the original list and insert it into a separate list of keys. We finally link the list of keys at the end of the remaining given list. The time complexity of this solution is also O(n) and it also requires only one traversal of the list.

Please refer complete article on Move all occurrences of an element to end in a linked list for more details!