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C++ Program To Check If Two Linked Lists Are Identical

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  • Last Updated : 03 Aug, 2022

Two Linked Lists are identical when they have the same data and the arrangement of data is also the same. For example, Linked lists a (1->2->3) and b(1->2->3) are identical. . Write a function to check if the given two linked lists are identical.

Method 1 (Iterative): 
To identify if two lists are identical, we need to traverse both lists simultaneously, and while traversing we need to compare data.


// An iterative C++ program to check if
// two linked lists are identical or not
using namespace std;
// Structure for a linked list node
struct Node
    int data;
    struct Node *next;
/* Returns true if linked lists a and b
   are identical, otherwise false */
bool areIdentical(struct Node *a,
                  struct Node *b)
    while (a != NULL && b != NULL)
        if (a->data != b->data)
            return false;
        /* If we reach here, then a and b are
           not NULL and their data is same, so
           move to next nodes in both lists */
        a = a->next;
        b = b->next;
    // If linked lists are identical, then
    // 'a' and 'b' must be NULL at this point.
    return (a == NULL && b == NULL);
   and fun2() */
/* Given a reference (pointer to pointer)
   to the head of a list and an int, push
   a new node on the front of the list. */
void push(struct Node** head_ref,
          int new_data)
    // Allocate node
    struct Node* new_node =
           (struct Node*) malloc(sizeof(struct Node));
    // Put in the data
    new_node->data = new_data;
    // Link the old list off the new node
    new_node->next = (*head_ref);
    // Move the head to point to the
    // new node
    (*head_ref) = new_node;
// Driver Code
int main()
    /* The constructed linked lists are :
       a: 3->2->1
       b: 3->2->1 */
    struct Node *a = NULL;
    struct Node *b = NULL;
    push(&a, 1);
    push(&a, 2);
    push(&a, 3);
    push(&b, 1);
    push(&b, 2);
    push(&b, 3);
    if(areIdentical(a, b))
        cout << "Identical";
        cout << "Not identical";
    return 0;
// This code is contributed by Akanksha Rai



Method 2 (Recursive): 
Recursive solution code is much cleaner than iterative code. You probably wouldn’t want to use the recursive version for production code, however, because it will use stack space which is proportional to the length of the lists.


// A recursive C++ function to check if two
// linked lists are identical or not
bool areIdentical(Node *a, Node *b)
    // If both lists are empty
    if (a == NULL && b == NULL)
    return true;
    // If both lists are not empty, then
    // data of current nodes must match,
    // and same should be recursively true
    // for rest of the nodes.
    if (a != NULL && b != NULL)
    return (a->data == b->data) &&
            areIdentical(a->next, b->next);
    // If we reach here, then one of the lists
    // is empty and other is not
    return false;
//This is code is contributed by rathbhupendra

Time Complexity: O(n) for both iterative and recursive versions. n is the length of the smaller list among a and b.

Auxiliary Space: O(n) for call stack because using recursion

Please refer complete article on Identical Linked Lists for more details!

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