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Find common elements in three linked lists
  • Difficulty Level : Medium
  • Last Updated : 22 Dec, 2020

Given three linked lists, find all common element among the three linked lists.

Examples:  

Input :  
   10 15 20 25 12
   10 12 13 15 
   10 12 15 24 25 26
Output : 10 12 15 

Input :
   1 2 3 4 5
   1 2 3 4 6 9 8
   1 2 4 5 10
Output : 1 2 4

Method 1 : (Simple) 
Use three-pointers to iterate the given three linked lists and if any element common print that element. 
Time complexity of the above solution will be O(N*N*N)

Method 2 : (Use Merge Sort) 
In this method, we first sort the three lists and then we traverse the sorted lists to get the intersection.
Following are the steps to be followed to get intersection of three lists:
1) Sort the first Linked List using merge sort. This step takes O(mLogm) time. Refer this post for details of this step.
2) Sort the second Linked List using merge sort. This step takes O(nLogn) time. Refer this post for details of this step.
3) Sort the third Linked List using merge sort. This step takes O(pLogp) time. Refer this post for details of this step.
3) Linearly scan three sorted lists to get the intersection. This step takes O(m + n + p) time. This step can be implemented using the same algorithm as sorted arrays algorithm discussed here.
Time complexity of this method is O(mLogm + nLogn + plogp) which is better than method 1’s time complexity.

Method 3 : (Hashing) 
Following are the steps to be followed to get intersection of three lists using hashing: 
1) Create an empty hash table. Iterate through the first linked list and mark all the element frequency as 1 in the hash table. This step takes O(m) time. 
2) Iterate through the second linked list and if current element frequency is 1 in hash table mark it as 2. This step takes O(n) time. 
3) Iterate the third linked list and if the current element frequency is 2 in hash table mark it as 3. This step takes O(p) time. 
4) Now iterate first linked list again to check the frequency of elements. if an element with frequency three exist in hash table, it will be present in the intersection of three linked lists. This step takes O(m) time. 
Time complexity of this method is O(m + n + p) which is better than time complexity of method 1 and 2.



Below is the implementation of the above idea.  

C++

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// C++ program to find common element
// in three unsorted linked list
#include <bits/stdc++.h>
#define max 1000000
using namespace std;
 
/* Link list node */
struct Node {
    int data;
    struct Node* next;
};
 
/* A utility function to insert a node at the
beginning of a linked list */
void push(struct Node** head_ref, int new_data)
{
    struct Node* new_node =
            (struct Node *)malloc(sizeof(struct Node));
    new_node->data = new_data;
    new_node->next = (*head_ref);
    (*head_ref) = new_node;
}
 
/* print the common element in between
given three linked list*/
void Common(struct Node* head1,
            struct Node* head2, struct Node* head3)
{
     
    // Creating empty hash table;
    unordered_map<int, int> hash;
     
    struct Node* p = head1;
    while (p != NULL) {
         
        // set frequency by 1
        hash[p->data] = 1;
        p = p->next;
    }
     
    struct Node* q = head2;
    while (q != NULL) {
         
        // if the element is already exist in the
        // linked list set its frequency 2
        if (hash.find(q->data) != hash.end())
            hash[q->data] = 2;
        q = q->next;
    }
     
    struct Node* r = head3;
    while (r != NULL) {
        if (hash.find(r->data) != hash.end() &&
            hash[r->data] == 2)
         
        // if the element frquancy is 2 it means
        // its present in both the first and second
        // linked list set its frquancy 3
        hash[r->data] = 3;
        r = r->next;
    }
 
     
    for (auto x : hash) {
         
        // if current frequency is 3 its means
        // element is common in all the given
        // linked list
        if (x.second == 3)
            cout << x.first << " ";
    }
}
 
// Driver code
int main()
{
 
    // first list
    struct Node* head1 = NULL;
    push(&head1, 20);
    push(&head1, 5);
    push(&head1, 15);
    push(&head1, 10);
 
    // second list
    struct Node* head2 = NULL;
    push(&head2, 10);
    push(&head2, 20);
    push(&head2, 15);
    push(&head2, 8);
         
    // third list
    struct Node* head3 = NULL;
    push(&head3, 10);
    push(&head3, 2);
    push(&head3, 15);
    push(&head3, 20);
 
    Common(head1, head2, head3);
     
    return 0;
}

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Java

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// Java program to find common element
// in three unsorted linked list
import java.util.*;
 
class GFG
{
static int max = 1000000;
 
/* Link list node */
static class Node
{
    int data;
    Node next;
};
 
/* A utility function to insert a node
at the beginning of a linked list */
static Node push(Node head_ref,
                 int new_data)
{
    Node new_node = new Node();
    new_node.data = new_data;
    new_node.next = head_ref;
    head_ref = new_node;
    return head_ref;
}
 
/* print the common element in between
given three linked list*/
static void Common(Node head1,
                   Node head2,
                   Node head3)
{
     
    // Creating empty hash table;
    HashMap<Integer,
            Integer> hash = new HashMap<Integer,
                                        Integer>();
     
    Node p = head1;
    while (p != null)
    {
         
        // set frequency by 1
        hash. put(p.data, 1);
        p = p.next;
    }
     
    Node q = head2;
    while (q != null)
    {
         
        // if the element is already exist in the
        // linked list set its frequency 2
        if (hash.containsKey(q.data))
            hash. put(q.data, 2);
        q = q.next;
    }
     
    Node r = head3;
    while (r != null)
    {
        if (hash.containsKey(r.data)&&
            hash.get(r.data) == 2)
         
        // if the element frquancy is 2 it means
        // its present in both the first and second
        // linked list set its frquancy 3
        hash. put(r.data, 3);
        r = r.next;
    }
 
    for (Map.Entry<Integer,
                   Integer> x : hash.entrySet())
    {
         
        // if current frequency is 3 its means
        // element is common in all the given
        // linked list
        if (x.getValue() == 3)
            System.out.println(x.getKey() + " ");
    }
}
 
// Driver code
public static void main(String[] args)
{
     
    // first list
    Node head1 = null;
    head1 = push(head1, 20);
    head1 = push(head1, 5);
    head1 = push(head1, 15);
    head1 = push(head1, 10);
 
    // second list
    Node head2 = null;
    head2 = push(head2, 10);
    head2 = push(head2, 20);
    head2 = push(head2, 15);
    head2 = push(head2, 8);
         
    // third list
    Node head3 = null;
    head3 = push(head3, 10);
    head3 = push(head3, 2);
    head3 = push(head3, 15);
    head3 = push(head3, 20);
 
    Common(head1, head2, head3);
}
}
 
// This code is contributed by 29AjayKumar

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Python3

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# Python3 program to find common element
# in three unsorted linked list
max = 1000000
 
# Link list node
class Node:
     
    def __init__(self, data):
         
        self.data = data
        self.next = None
         
# A utility function to insert a node at the
# beginning of a linked list
def push( head_ref, new_data):
     
    new_node = Node(new_data)
    new_node.next = head_ref
    head_ref = new_node
    return head_ref
 
# Print the common element in between
# given three linked list
def Common(head1, head2, head3):
      
    # Creating empty hash table;
    hash = dict()
      
    p = head1
    while (p != None):
         
        # Set frequency by 1
        hash[p.data] = 1
        p = p.next
     
    q = head2
     
    while (q != None):
         
        # If the element is already exist in the
        # linked list set its frequency 2
        if (q.data in hash):
            hash[q.data] = 2
             
        q = q.next
 
    r = head3
     
    while (r != None):
        if (r.data in hash) and hash[r.data] == 2:
             
            # If the element frquancy is 2 it means
            # its present in both the first and second
            # linked list set its frquancy 3
            hash[r.data] = 3
             
        r = r.next
     
    for x in hash.keys():
         
        # If current frequency is 3 its means
        # element is common in all the given
        # linked list
        if (hash[x] == 3):
            print(x, end = ' ')
             
# Driver code
if __name__=='__main__':
  
    # First list
    head1 = None
    head1 = push(head1, 20)
    head1 = push(head1, 5)
    head1 = push(head1, 15)
    head1 = push(head1, 10)
  
    # Second list
    head2 = None
    head2 = push(head2, 10)
    head2 = push(head2, 20)
    head2 = push(head2, 15)
    head2 = push(head2, 8)
          
    # Third list
    head3 = None
    head3 = push(head3, 10)
    head3 = push(head3, 2)
    head3 = push(head3, 15)
    head3 = push(head3, 20)
  
    Common(head1, head2, head3)
      
# This code is contributed by rutvik_56

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C#

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// C# program to find common element
// in three unsorted linked list
using System;
using System.Collections.Generic;
 
class GFG
{
static int max = 1000000;
 
/* Link list node */
public class Node
{
    public int data;
    public Node next;
};
 
/* A utility function to insert a node
at the beginning of a linked list */
static Node push(Node head_ref,
                int new_data)
{
    Node new_node = new Node();
    new_node.data = new_data;
    new_node.next = head_ref;
    head_ref = new_node;
    return head_ref;
}
 
/* print the common element in between
given three linked list*/
static void Common(Node head1,
                Node head2,
                Node head3)
{
     
    // Creating empty hash table;
    Dictionary<int,    
               int> hash = new Dictionary<int,
                                          int>();
     
    Node p = head1;
    while (p != null)
    {
         
        // set frequency by 1
        hash.Add(p.data, 1);
        p = p.next;
    }
     
    Node q = head2;
    while (q != null)
    {
         
        // if the element is already exist in the
        // linked list set its frequency 2
        if (hash.ContainsKey(q.data))
            hash[q.data] = 2;
        q = q.next;
    }
     
    Node r = head3;
    while (r != null)
    {
        if (hash.ContainsKey(r.data)&&
            hash[r.data] == 2)
         
        // if the element frquancy is 2 it means
        // its present in both the first and
        // second linked list set its frquancy 3
        hash[r.data] = 3;
        r = r.next;
    }
 
    foreach(KeyValuePair<int, int> x in hash)
    {
         
        // if current frequency is 3 its means
        // element is common in all the given
        // linked list
        if (x.Value == 3)
            Console.Write(x.Key + " ");
    }
}
 
// Driver code
public static void Main(String[] args)
{
     
    // first list
    Node head1 = null;
    head1 = push(head1, 20);
    head1 = push(head1, 5);
    head1 = push(head1, 15);
    head1 = push(head1, 10);
 
    // second list
    Node head2 = null;
    head2 = push(head2, 10);
    head2 = push(head2, 20);
    head2 = push(head2, 15);
    head2 = push(head2, 8);
         
    // third list
    Node head3 = null;
    head3 = push(head3, 10);
    head3 = push(head3, 2);
    head3 = push(head3, 15);
    head3 = push(head3, 20);
 
    Common(head1, head2, head3);
}
}
 
// This code is contributed by Princi Singh

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Output: 

10 15 20

 

Time Complexity : O(m + n + p)
 

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