# Multiply two numbers represented as linked lists into a third list

Given two numbers represented by linked lists, write a function that returns the head of the new linked list that represents the number that is the product of those numbers.

Examples:

```Input : 9->4->6
8->4
Output : 7->9->4->6->4

Input : 9->9->9->4->6->9
9->9->8->4->9
Output : 9->9->7->9->5->9->8->0->1->8->1
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

We have already discussed a solution in below post.
Multiply two numbers represented by Linked Lists

The solution discussed above store result in an integer. Her we store result in a third list so that large numbers can be handled.

Remember old school multiplication? we imitate that process. On paper, we take the last digit of a number and multiply with the second number and write the product. Now leave the last column and same way each digit of one number is multiplied with every digit of other number and every time result is written by leaving one last column. then add these columns that forms the number. Now assume these columns as nodes of the resultant linked list. We make resultant linked list in reversed fashion.

Algorithm

```Reverse both linked lists
Make a linked list of maximum result size (m + n + 1)
For each node of one list
For each node of second list
a) Multiply nodes
b) Add digit in result LL at corresponding
position
c) Now resultant node itself can be higher
than one digit
d) Make carry for next node
Leave one last column means next time start
From next node in result list
```

## C++

 `// C++ program to Multiply two numbers  ` `// represented as linked lists  ` `#include   ` `using` `namespace` `std; ` ` `  `// Linked list Node  ` `struct` `Node {  ` `    ``int` `data;  ` `    ``struct` `Node* next;  ` `};  ` ` `  `// Function to create a new Node  ` `// with given data  ` `struct` `Node* newNode(``int` `data)  ` `{  ` `    ``struct` `Node* new_node =  ` `        ``(``struct` `Node*)``malloc``(``sizeof``(``struct` `Node));  ` `    ``new_node->data = data;  ` `    ``new_node->next = NULL;  ` `    ``return` `new_node;  ` `}  ` ` `  `// Function to insert a Node at the  ` `// beginning of the Linked List  ` `void` `push(``struct` `Node** head_ref, ``int` `new_data)  ` `{  ` `    ``// allocate Node  ` `    ``struct` `Node* new_node = newNode(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;  ` `}  ` ` `  `// Function to reverse the linked list and return  ` `// its length  ` `int` `reverse(``struct` `Node** head_ref)  ` `{  ` `    ``struct` `Node* prev = NULL;  ` `    ``struct` `Node* current = *head_ref;  ` `    ``struct` `Node* next;  ` `    ``int` `len = 0;  ` `    ``while` `(current != NULL) {  ` `        ``len++;  ` `        ``next = current->next;  ` `        ``current->next = prev;  ` `        ``prev = current;  ` `        ``current = next;  ` `    ``}  ` `    ``*head_ref = prev;  ` `    ``return` `len;  ` `}  ` ` `  `// Function to make an empty linked list of  ` `// given size  ` `struct` `Node* make_empty_list(``int` `size)  ` `{  ` `    ``struct` `Node* head = NULL;  ` `    ``while` `(size--)  ` `        ``push(&head, 0);  ` `    ``return` `head;  ` `}  ` ` `  `// Multiply contents of two linked lists => store  ` `// in another list and return its head  ` `struct` `Node* multiplyTwoLists(``struct` `Node* first,  ` `                        ``struct` `Node* second)  ` `{  ` `    ``// reverse the lists to muliply from end  ` `    ``// m and n lengths of linked lists to make  ` `    ``// and empty list  ` `    ``int` `m = reverse(&first), n = reverse(&second);  ` ` `  `    ``// make a list that will contain the result  ` `    ``// of multiplication.  ` `    ``// m+n+1 can be max size of the list  ` `    ``struct` `Node* result = make_empty_list(m + n + 1);  ` ` `  `    ``// pointers for traverse linked lists and also  ` `    ``// to reverse them after  ` `    ``struct` `Node *second_ptr = second,  ` `        ``*result_ptr1 = result, *result_ptr2, *first_ptr;  ` ` `  `    ``// multiply each Node of second list with first  ` `    ``while` `(second_ptr) {  ` ` `  `        ``int` `carry = 0;  ` ` `  `        ``// each time we start from the next of Node  ` `        ``// from which we started last time  ` `        ``result_ptr2 = result_ptr1;  ` ` `  `        ``first_ptr = first;  ` ` `  `        ``while` `(first_ptr) {  ` ` `  `            ``// multiply a first list's digit with a  ` `            ``// current second list's digit  ` `            ``int` `mul = first_ptr->data * second_ptr->data  ` `                    ``+ carry;  ` ` `  `            ``// Assigne the product to corresponding Node  ` `            ``// of result  ` `            ``result_ptr2->data += mul % 10;  ` ` `  `            ``// now resultant Node itself can have more  ` `            ``// than 1 digit  ` `            ``carry = mul / 10 + result_ptr2->data / 10;  ` `            ``result_ptr2->data = result_ptr2->data % 10;  ` ` `  `            ``first_ptr = first_ptr->next;  ` `            ``result_ptr2 = result_ptr2->next;  ` `        ``}  ` ` `  `        ``// if carry is remaining from last multiplication  ` `        ``if` `(carry > 0) {  ` `            ``result_ptr2->data += carry;  ` `        ``}  ` ` `  `        ``result_ptr1 = result_ptr1->next;  ` `        ``second_ptr = second_ptr->next;  ` `    ``}  ` ` `  `    ``// reverse the result_list as it was populated  ` `    ``// from last Node  ` `    ``reverse(&result);  ` `    ``reverse(&first);  ` `    ``reverse(&second);  ` ` `  `    ``// remove if there are zeros at starting  ` `    ``while` `(result->data == 0) {  ` `        ``struct` `Node* temp = result;  ` `        ``result = result->next;  ` `        ``free``(temp);  ` `    ``}  ` ` `  `    ``// Return head of multiplication list  ` `    ``return` `result;  ` `}  ` ` `  `// A utility function to print a linked list  ` `void` `printList(``struct` `Node* Node)  ` `{  ` `    ``while` `(Node != NULL) {  ` `        ``cout << Node->data;  ` `        ``if` `(Node->next)  ` `            ``cout<<``"->"``;  ` `        ``Node = Node->next;  ` `    ``}  ` `    ``cout << endl;  ` `}  ` ` `  `// Driver program to test above function  ` `int` `main(``void``)  ` `{  ` `    ``struct` `Node* first = NULL;  ` `    ``struct` `Node* second = NULL;  ` ` `  `    ``// create first list 9->9->9->4->6->9  ` `    ``push(&first, 9);  ` `    ``push(&first, 6);  ` `    ``push(&first, 4);  ` `    ``push(&first, 9);  ` `    ``push(&first, 9);  ` `    ``push(&first, 9);  ` `    ``cout<<``"First List is: "``;  ` `    ``printList(first);  ` ` `  `    ``// create second list 9->9->8->4->9  ` `    ``push(&second, 9);  ` `    ``push(&second, 4);  ` `    ``push(&second, 8);  ` `    ``push(&second, 9);  ` `    ``push(&second, 9);  ` `    ``cout<<``"Second List is: "``;  ` `    ``printList(second);  ` ` `  `    ``// Multiply the two lists and see result  ` `    ``struct` `Node* result = multiplyTwoLists(first, second);  ` `    ``cout << ``"Resultant list is: "``;  ` `    ``printList(result);  ` ` `  `    ``return` `0;  ` `}  ` ` `  `// This code is contributed by SHUBHAMSINGH10 `

## C

 `// C program to Multiply two numbers ` `// represented as linked lists ` `#include ` `#include ` ` `  `// Linked list Node ` `struct` `Node { ` `    ``int` `data; ` `    ``struct` `Node* next; ` `}; ` ` `  `// Function to create a new Node ` `// with given data ` `struct` `Node* newNode(``int` `data) ` `{ ` `    ``struct` `Node* new_node = ` `        ``(``struct` `Node*)``malloc``(``sizeof``(``struct` `Node)); ` `    ``new_node->data = data; ` `    ``new_node->next = NULL; ` `    ``return` `new_node; ` `} ` ` `  `// Function to insert a Node at the ` `// beginning of the Linked List ` `void` `push(``struct` `Node** head_ref, ``int` `new_data) ` `{ ` `    ``// allocate Node ` `    ``struct` `Node* new_node = newNode(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; ` `} ` ` `  `// Function to reverse the linked list and return ` `// its length ` `int` `reverse(``struct` `Node** head_ref) ` `{ ` `    ``struct` `Node* prev = NULL; ` `    ``struct` `Node* current = *head_ref; ` `    ``struct` `Node* next; ` `    ``int` `len = 0; ` `    ``while` `(current != NULL) { ` `        ``len++; ` `        ``next = current->next; ` `        ``current->next = prev; ` `        ``prev = current; ` `        ``current = next; ` `    ``} ` `    ``*head_ref = prev; ` `    ``return` `len; ` `} ` ` `  `// Function to make an empty linked list of ` `// given size ` `struct` `Node* make_empty_list(``int` `size) ` `{ ` `    ``struct` `Node* head = NULL; ` `    ``while` `(size--) ` `        ``push(&head, 0); ` `    ``return` `head; ` `} ` ` `  `// Multiply contents of two linked lists => store ` `// in another list and return its head ` `struct` `Node* multiplyTwoLists(``struct` `Node* first, ` `                           ``struct` `Node* second) ` `{ ` `    ``// reverse the lists to muliply from end ` `    ``// m and n lengths of linked lists to make ` `    ``// and empty list ` `    ``int` `m = reverse(&first), n = reverse(&second); ` ` `  `    ``// make a list that will contain the result ` `    ``// of multiplication. ` `    ``// m+n+1 can be max size of the list ` `    ``struct` `Node* result = make_empty_list(m + n + 1); ` ` `  `    ``// pointers for traverse linked lists and also ` `    ``// to reverse them after ` `    ``struct` `Node *second_ptr = second, ` `          ``*result_ptr1 = result, *result_ptr2, *first_ptr; ` ` `  `    ``// multiply each Node of second list with first ` `    ``while` `(second_ptr) { ` ` `  `        ``int` `carry = 0; ` ` `  `        ``// each time we start from the next of Node ` `        ``// from which we started last time ` `        ``result_ptr2 = result_ptr1; ` ` `  `        ``first_ptr = first; ` ` `  `        ``while` `(first_ptr) { ` ` `  `            ``// multiply a first list's digit with a ` `            ``// current second list's digit ` `            ``int` `mul = first_ptr->data * second_ptr->data ` `                      ``+ carry; ` ` `  `            ``// Assigne the product to corresponding Node ` `            ``// of result ` `            ``result_ptr2->data += mul % 10; ` ` `  `            ``// now resultant Node itself can have more ` `            ``// than 1 digit ` `            ``carry = mul / 10 + result_ptr2->data / 10; ` `            ``result_ptr2->data = result_ptr2->data % 10; ` ` `  `            ``first_ptr = first_ptr->next; ` `            ``result_ptr2 = result_ptr2->next; ` `        ``} ` ` `  `        ``// if carry is remaining from last multiplication ` `        ``if` `(carry > 0) { ` `            ``result_ptr2->data += carry; ` `        ``} ` ` `  `        ``result_ptr1 = result_ptr1->next; ` `        ``second_ptr = second_ptr->next; ` `    ``} ` ` `  `    ``// reverse the result_list as it was populated ` `    ``// from last Node ` `    ``reverse(&result); ` `    ``reverse(&first); ` `    ``reverse(&second); ` ` `  `    ``// remove if there are zeros at starting ` `    ``while` `(result->data == 0) { ` `        ``struct` `Node* temp = result; ` `        ``result = result->next; ` `        ``free``(temp); ` `    ``} ` ` `  `    ``// Return head of multiplication list ` `    ``return` `result; ` `} ` ` `  `// A utility function to print a linked list ` `void` `printList(``struct` `Node* Node) ` `{ ` `    ``while` `(Node != NULL) { ` `        ``printf``(``"%d"``, Node->data); ` `        ``if` `(Node->next) ` `            ``printf``(``"->"``); ` `        ``Node = Node->next; ` `    ``} ` `    ``printf``(``"\n"``); ` `} ` ` `  `// Driver program to test above function ` `int` `main(``void``) ` `{ ` `    ``struct` `Node* first = NULL; ` `    ``struct` `Node* second = NULL; ` ` `  `    ``// create first list 9->9->9->4->6->9 ` `    ``push(&first, 9); ` `    ``push(&first, 6); ` `    ``push(&first, 4); ` `    ``push(&first, 9); ` `    ``push(&first, 9); ` `    ``push(&first, 9); ` `    ``printf``(``"First List is: "``); ` `    ``printList(first); ` ` `  `    ``// create second list 9->9->8->4->9 ` `    ``push(&second, 9); ` `    ``push(&second, 4); ` `    ``push(&second, 8); ` `    ``push(&second, 9); ` `    ``push(&second, 9); ` `    ``printf``(``"Second List is: "``); ` `    ``printList(second); ` ` `  `    ``// Multiply the two lists and see result ` `    ``struct` `Node* result = multiplyTwoLists(first, second); ` `    ``printf``(``"Resultant list is: "``); ` `    ``printList(result); ` ` `  `    ``return` `0; ` `} `

Output:

```First List is: 9->9->9->4->6->9
Second List is: 9->9->8->4->9
Resultant list is: 9->9->7->9->5->9->8->0->1->8->1
```

Note: we can take care of resultant node that can have more than 1 digit outside the loop just traverse the result list and add carry to next digit before reversing.

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Improved By : SHUBHAMSINGH10

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