Find common elements in three linked lists

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 C++ implementation of the above idea.

// 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;
}
Output:

10 15 20

Time Complexity : O(m + n + p)




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