Sublist Search (Search a linked list in another list)

Given two linked lists, the task is to check whether the first list is present in 2nd list or not. 

Examples:

Input  : list1 =  10->20
         list2  = 5->10->20
Output : LIST FOUND

Input  : list1 =  1->2->3->4
         list2  = 1->2->1->2->3->4
Output : LIST FOUND

Input  : list1 =  1->2->3->4
         list2  = 1->2->2->1->2->3
Output : LIST NOT FOUND

Algorithm: 

1. Take first node of second list. 
2. Start matching the first list from this first node. 
3. If whole lists match return true. 
4. Else break and take first list to the first node again. 
5. And take second list to its second node. 
6. Repeat these steps until any of linked lists becomes empty. 
7. If first list becomes empty then list found else not.

Below is the implementation.

C++

// C++ program to find a list in second list
#include <bits/stdc++.h>
using namespace std;

// A Linked List node
struct Node
{
    int data;
    Node* next;
};

// Returns true if first list is present in second
// list
bool findList(Node* first, Node* second)
{
    Node* ptr1 = first, *ptr2 = second;

    // If both linked lists are empty, return true
    if (first == NULL && second == NULL)
        return true;

    // Else If one is empty and other is not return
    // false
    if ( first == NULL ||
        (first != NULL && second == NULL))
        return false;

    // Traverse the second list by picking nodes
    // one by one
    while (second != NULL)
    {
        // Initialize ptr2 with current node of second
        ptr2 = second;

        // Start matching first list with second list
        while (ptr1 != NULL)
        {
            // If second list becomes empty and first
            // not then return false
            if (ptr2 == NULL)
                return false;

            // If data part is same, go to next
            // of both lists
            else if (ptr1->data == ptr2->data)
            {
                ptr1 = ptr1->next;
                ptr2 = ptr2->next;
            }

            // If not equal then  break the loop
            else break;
        }

        // Return true if first list gets traversed
        // completely that means it is matched.
        if (ptr1 == NULL)
            return true;

        // Initialize ptr1 with first again
        ptr1 = first;

        // And go to next node of second list
        second = second->next;
    }

    return false;
}

/* Function to print nodes in a given linked list */
void printList(Node* node)
{
    while (node != NULL)
    {
        printf("%d ", node->data);
        node = node->next;
    }
}

// Function to add new node to linked lists
Node *newNode(int key)
{
    Node *temp = new Node;
    temp-> data= key;
    temp->next = NULL;
    return temp;
}

/* Driver program to test above functions*/
int main()
{
    /* Let us create two linked lists to test
    the above functions. Created lists shall be
        a: 1->2->3->4
        b: 1->2->1->2->3->4*/
    Node *a = newNode(1);
    a->next = newNode(2);
    a->next->next = newNode(3);
    a->next->next->next = newNode(4);

    Node *b = newNode(1);
    b->next = newNode(2);
    b->next->next = newNode(1);
    b->next->next->next = newNode(2);
    b->next->next->next->next = newNode(3);
    b->next->next->next->next->next = newNode(4);

    findList(a,b) ? cout << "LIST FOUND" :
                    cout << "LIST NOT FOUND";

    return 0;
}

Java

// Java program to find a list in second list
import java.util.*;
class GFG
{

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

// Returns true if first list is
// present in second list
static boolean findList(Node first,
                        Node second)
{
    Node ptr1 = first, ptr2 = second;

    // If both linked lists are empty,
    // return true
    if (first == null && second == null)
        return true;

    // Else If one is empty and
    // other is not, return false
    if (first == null ||
       (first != null && second == null))
        return false;

    // Traverse the second list by
    // picking nodes one by one
    while (second != null)
    {
        // Initialize ptr2 with
        // current node of second
        ptr2 = second;

        // Start matching first list
        // with second list
        while (ptr1 != null)
        {
            // If second list becomes empty and
            // first not then return false
            if (ptr2 == null)
                return false;

            // If data part is same, go to next
            // of both lists
            else if (ptr1.data == ptr2.data)
            {
                ptr1 = ptr1.next;
                ptr2 = ptr2.next;
            }

            // If not equal then break the loop
            else break;
        }

        // Return true if first list gets traversed
        // completely that means it is matched.
        if (ptr1 == null)
            return true;

        // Initialize ptr1 with first again
        ptr1 = first;

        // And go to next node of second list
        second = second.next;
    }
    return false;
}

/* Function to print nodes in a given linked list */
static void printList(Node node)
{
    while (node != null)
    {
        System.out.printf("%d ", node.data);
        node = node.next;
    }
}

// Function to add new node to linked lists
static Node newNode(int key)
{
    Node temp = new Node();
    temp.data= key;
    temp.next = null;
    return temp;
}

// Driver Code
public static void main(String[] args)
{
    /* Let us create two linked lists to test
    the above functions. Created lists shall be
        a: 1->2->3->4
        b: 1->2->1->2->3->4*/
    Node a = newNode(1);
    a.next = newNode(2);
    a.next.next = newNode(3);
    a.next.next.next = newNode(4);

    Node b = newNode(1);
    b.next = newNode(2);
    b.next.next = newNode(1);
    b.next.next.next = newNode(2);
    b.next.next.next.next = newNode(3);
    b.next.next.next.next.next = newNode(4);

    if(findList(a, b) == true)
        System.out.println("LIST FOUND");
    else
        System.out.println("LIST NOT FOUND");
}
}

// This code is contributed by Princi Singh

Python3

# Python3 program to find a list in second list
class Node:
    def __init__(self, value = 0):

        self.value = value
        self.next = None

# Returns true if first list is
# present in second list
def findList(first, second):

    # If both linked lists are empty/None,
    # return True
    if not first and not second:
        return True

    # If ONLY one of them is empty,
    # return False
    if not first or not second:
        return False

    ptr1 = first
    ptr2 = second

    # Traverse the second LL by
    # picking nodes one by one
    while ptr2:

        # Initialize 'ptr2' with current
        # node of 'second'
        ptr2 = second

        # Start matching first LL
        # with second LL
        while ptr1:

            # If second LL become empty and
            # first not, return False,
            # since first LL has not been
            # traversed completely
            if not ptr2:
                return False

            # If value of both nodes from both
            # LLs are equal, increment pointers
            # for both LLs so that next value
            # can be matched
            else if ptr1.value == ptr2.value:
                ptr1 = ptr1.next
                ptr2 = ptr2.next

            # If a single mismatch is found
            # OR ptr1 is None/empty,break out
            # of the while loop and do some checks
            else:
                break

        # check 1 :
        # If 'ptr1' is None/empty,that means
        # the 'first LL' has been completely
        # traversed and matched so return True
        if not ptr1:
            return True

        # If check 1 fails, that means, some
        # items for 'first' LL are still yet
        # to be matched, so start again by
        # bringing back the 'ptr1' to point
        # to 1st node of 'first' LL
        ptr1 = first

        # And increment second node element to next
        second = second.next

    return False

# Driver Code

# Let us create two linked lists to
# test the above functions.
# Created lists would be be
# node_a: 1->2->3->4
# node_b: 1->2->1->2->3->4
node_a = Node(1)
node_a.next = Node(2)
node_a.next.next = Node(3)
node_a.next.next.next = Node(4)

node_b = Node(1)
node_b.next = Node(2)
node_b.next.next = Node(1)
node_b.next.next.next = Node(2)
node_b.next.next.next.next = Node(3)
node_b.next.next.next.next.next = Node(4)

if findList(node_a, node_b):
    print("LIST FOUND")
else:
    print("LIST NOT FOUND")

# This code is contributed by GauriShankarBadola

C#

// C# program to find a list in second list
using System;

class GFG
{

// A Linked List node
class Node
{
    public int data;
    public Node next;
};

// Returns true if first list is
// present in second list
static Boolean findList(Node first,
                        Node second)
{
    Node ptr1 = first, ptr2 = second;

    // If both linked lists are empty,
    // return true
    if (first == null && second == null)
        return true;

    // Else If one is empty and
    // other is not, return false
    if (first == null ||
       (first != null && second == null))
        return false;

    // Traverse the second list by
    // picking nodes one by one
    while (second != null)
    {
        // Initialize ptr2 with
        // current node of second
        ptr2 = second;

        // Start matching first list
        // with second list
        while (ptr1 != null)
        {
            // If second list becomes empty and
            // first not then return false
            if (ptr2 == null)
                return false;

            // If data part is same, go to next
            // of both lists
            else if (ptr1.data == ptr2.data)
            {
                ptr1 = ptr1.next;
                ptr2 = ptr2.next;
            }

            // If not equal then break the loop
            else break;
        }

        // Return true if first list gets traversed
        // completely that means it is matched.
        if (ptr1 == null)
            return true;

        // Initialize ptr1 with first again
        ptr1 = first;

        // And go to next node of second list
        second = second.next;
    }
    return false;
}

/* Function to print nodes
in a given linked list */
static void printList(Node node)
{
    while (node != null)
    {
        Console.Write("{0} ", node.data);
        node = node.next;
    }
}

// Function to add new node to linked lists
static Node newNode(int key)
{
    Node temp = new Node();
    temp.data= key;
    temp.next = null;
    return temp;
}

// Driver Code
public static void Main(String[] args)
{
    /* Let us create two linked lists to test
    the above functions. Created lists shall be
        a: 1->2->3->4
        b: 1->2->1->2->3->4*/
    Node a = newNode(1);
    a.next = newNode(2);
    a.next.next = newNode(3);
    a.next.next.next = newNode(4);

    Node b = newNode(1);
    b.next = newNode(2);
    b.next.next = newNode(1);
    b.next.next.next = newNode(2);
    b.next.next.next.next = newNode(3);
    b.next.next.next.next.next = newNode(4);

    if(findList(a, b) == true)
        Console.Write("LIST FOUND");
    else
        Console.Write("LIST NOT FOUND");
}
}

// This code is contributed by Rajput-Ji

Javascript

<script>

// JavaScript program to find a
// list in second list

    // A Linked List node
    class Node {
    constructor() {
        this.data = 0;
        this.next = null;
    }
}

    // Returns true if first list is
    // present in second list
    function findList(first,  second) {
    var ptr1 = first, ptr2 = second;

        // If both linked lists are empty,
        // return true
        if (first == null && second == null)
            return true;

        // Else If one is empty and
        // other is not, return false
        if (first == null || (first != null &&
        second == null))
            return false;

        // Traverse the second list by
        // picking nodes one by one
        while (second != null) {
            // Initialize ptr2 with
            // current node of second
            ptr2 = second;

            // Start matching first list
            // with second list
            while (ptr1 != null) {
                // If second list becomes empty and
                // first not then return false
                if (ptr2 == null)
                    return false;

                // If data part is same, go to next
                // of both lists
                else if (ptr1.data == ptr2.data) {
                    ptr1 = ptr1.next;
                    ptr2 = ptr2.next;
                }

                // If not equal then break the loop
                else
                    break;
            }

            // Return true if first list gets traversed
            // completely that means it is matched.
            if (ptr1 == null)
                return true;

            // Initialize ptr1 with first again
            ptr1 = first;

            // And go to next node of second list
            second = second.next;
        }
        return false;
    }

    /* Function to print nodes in a given linked list */
    function printList(node) {
        while (node != null) {
            document.write("%d ", node.data);
            node = node.next;
        }
    }

    // Function to add new node to linked lists
    function newNode(key) {
        var temp = new Node();
        temp.data = key;
        temp.next = null;
        return temp;
    }

    // Driver Code

        /*
         Let us create two linked lists to test
         the above functions. Created lists
         shall be a: 1->2->3->4 b: 1->2->1->2->3->4
         */
        var a = newNode(1);
        a.next = newNode(2);
        a.next.next = newNode(3);
        a.next.next.next = newNode(4);

        var b = newNode(1);
        b.next = newNode(2);
        b.next.next = newNode(1);
        b.next.next.next = newNode(2);
        b.next.next.next.next = newNode(3);
        b.next.next.next.next.next = newNode(4);

        if (findList(a, b) == true)
            document.write("LIST FOUND");
        else
            document.write("LIST NOT FOUND");

// This code contributed by gauravrajput1

</script>

Output

LIST FOUND

Time Complexity: O(m*n) where m is the number of nodes in second list and n in first.

Optimization : 

Above code can be optimized by using extra space i.e. stores the list into two strings and apply KMP algorithm. Refer https://ide.geeksforgeeks.org/3fXUaV for implementation provided by Nishant Singh

Check out DSA Self Paced Course

This article is contributed by Sahil Chhabra (akku). If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.