C Program To Add Two Numbers Represented By Linked Lists- Set 1

Given two numbers represented by two lists, write a function that returns the sum list. The sum list is a list representation of the addition of two input numbers.

Example:

Input: 
List1: 5->6->3 // represents number 563 
List2: 8->4->2 // represents number 842 
Output: 
Resultant list: 1->4->0->5 // represents number 1405 
Explanation: 563 + 842 = 1405

Input: 
List1: 7->5->9->4->6 // represents number 75946
List2: 8->4 // represents number 84
Output: 
Resultant list: 7->6->0->3->0// represents number 76030
Explanation: 75946+84=76030

Approach: Traverse both lists and One by one pick nodes of both lists and add the values. If the sum is more than 10 then make carry as 1 and reduce sum. If one list has more elements than the other then consider the remaining values of this list as 0. The steps are:

1. Traverse the two linked lists from start to end
2. Add the two digits each from respective linked lists.
3. If one of the lists has reached the end then take 0 as its digit.
4. Continue it until both the end of the lists.
5. If the sum of two digits is greater than 9 then set carry as 1 and the current digit as sum % 10

Below is the implementation of this approach. 

C

// C program to implement the
// above approach
#include <stdio.h>
#include <stdlib.h>

// 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 Singly Linked List */
void push(struct Node** head_ref,
          int new_data)
{
    // Allocate node
    struct Node* new_node = newNode(new_data);

    // Link the old list of the new node
    new_node->next = (*head_ref);

    // Move the head to point to the
    // new node
    (*head_ref) = new_node;
}

/* Adds contents of two linked
   lists and return the head node
   of resultant list */
struct Node* addTwoLists(struct Node* first,
                         struct Node* second)
{
    // res is head node of the resultant
    // list
    struct Node* res = NULL;
    struct Node *temp, *prev = NULL;
    int carry = 0, sum;

    // while both lists exist
    while (first != NULL ||
           second != NULL)
    {
        // Calculate value of next digit in
        // resultant list. The next digit is
        // sum of following things
        // (i)  Carry
        // (ii) Next digit of first
        // list (if there is a next digit)
        // (ii) Next digit of second
        // list (if there is a next digit)
        sum = carry + (first ? first->data : 0) +
              (second ? second->data : 0);

        // Update carry for next calculation
        carry = (sum >= 10) ? 1 : 0;

        // Update sum if it is greater than 10
        sum = sum % 10;

        // Create a new node with sum as data
        temp = newNode(sum);

        // If this is the first node then
        // set it as head of the resultant
        // list
        if (res == NULL)
            res = temp;

        // If this is not the first node
        // then connect it to the rest.
        else
            prev->next = temp;

        // Set prev for next insertion
        prev = temp;

        // Move first and second pointers to
        // next nodes
        if (first)
            first = first->next;
        if (second)
            second = second->next;
    }

    if (carry > 0)
        temp->next = newNode(carry);

    // return head of the resultant list
    return res;
}

// A utility function to print a
// linked list
void printList(struct Node* node)
{
    while (node != NULL)
    {
        printf("%d ", node->data);
        node = node->next;
    }
    printf("");
}

// Driver code
int main(void)
{
    struct Node* res = NULL;
    struct Node* first = NULL;
    struct Node* second = NULL;

    // Create first list 7->5->9->4->6
    push(&first, 6);
    push(&first, 4);
    push(&first, 9);
    push(&first, 5);
    push(&first, 7);
    printf("First List is ");
    printList(first);

    // Create second list 8->4
    push(&second, 4);
    push(&second, 8);
    printf("Second List is ");
    printList(second);

    // Add the two lists and see result
    res = addTwoLists(first, second);
    printf("Resultant list is ");
    printList(res);

    return 0;
}

Output:

First List is 7 5 9 4 6 
Second List is 8 4 
Resultant list is 5 0 0 5 6 

Complexity Analysis:

• Time Complexity: O(m + n), where m and n are numbers of nodes in first and second lists respectively. 
  The lists need to be traversed only once.
• Auxiliary Space: O(1). 
  

Please refer complete article on Add two numbers represented by linked lists | Set 1 for more details!