Subtract Two Numbers represented as Linked Lists

Given two linked lists that represent two large positive numbers. Subtract the smaller number from larger one and return the difference as a linked list. Note that the input lists may be in any order, but we always need to subtract smaller from larger one.

It may be assumed that there are no extra leading zeros in input lists.

Examples

Input  : l1 = 1 -> 0 -> 0 -> NULL,  l2 = 1 -> NULL
Output : 0->9->9->NULL

Input  : l1 = 1 -> 0 -> 0 -> NULL,  l2 = 1 -> NULL
Output : 0->9->9->NULL

Input  : l1 = 7-> 8 -> 6 -> NULL,  l2 = 7 -> 8 -> 9 NULL
Output : 3->NULL

Following are the steps.
1) Calculate sizes of given two linked lists.
2) If sizes not are same, then append zeros in smaller linked list.
3) If size are same, then follow below steps:
….a) Find the smaller valued linked list.
….b) One by one subtract nodes of smaller sized linked list from larger size. Keep track of borrow while subtracting.

Following is C implementation of the above approach.

// C++ program to subtract smaller valued list from
// larger valued list and return result as a list.
#include<bits/stdc++.h>
using namespace std;

// A linked List Node
struct Node
{
    int data;
    struct Node* next;
};

// A utility which creates Node.
Node* newNode(int data)
{
    Node* temp = new Node;
    temp->data = data;
    temp->next = NULL;
    return temp;
}

/* A utility function to get length of linked list */
int getLength(Node *Node)
{
    int size = 0;
    while (Node != NULL)
    {
        Node = Node->next;
        size++;
    }
    return size;
}

/* A Utility that padds zeros in front of the
   Node, with the given diff */
Node* paddZeros(Node* sNode, int diff)
{
    if (sNode == NULL)
        return NULL;

    Node* zHead = newNode(0);
    diff--;
    Node* temp = zHead;
    while (diff--)
    {
        temp->next = newNode(0);
        temp = temp->next;
    }
    temp->next = sNode;
    return zHead;
}

/* Subtract LinkedList Helper is a recursive function,
   move till the last Node,  and subtract the digits and
   create the Node and return the Node. If d1 < d2, we
   borrow the number from previous digit. */
Node* subtractLinkedListHelper(Node* l1, Node* l2, bool& borrow)
{
    if (l1 == NULL && l2 == NULL && borrow == 0)
        return NULL;

    Node* previous = subtractLinkedListHelper(l1 ? l1->next : NULL,
                                    l2 ? l2->next : NULL, borrow);

    int d1 = l1->data;
    int d2 = l2->data;
    int sub = 0;

    /* if you have given the value value to next digit then
       reduce the d1 by 1 */
    if (borrow)
    {
        d1--;
        borrow = false;
    }

    /* If d1 < d2 , then borrow the number from previous digit.
       Add 10 to d1 and set borrow = true; */
    if (d1 < d2)
    {
        borrow = true;
        d1 = d1 + 10;
    }

    /* subtract the digits */
    sub = d1 - d2;

    /* Create a Node with sub value */
    Node* current = newNode(sub);

    /* Set the Next pointer as Previous */
    current->next = previous;

    return current;
}

/* This API subtracts two linked lists and returns the
   linked list which shall  have the subtracted result. */
Node* subtractLinkedList(Node* l1, Node* l2)
{
    // Base Case.
    if (l1 == NULL &&  l2 == NULL)
        return NULL;

    // In either of the case, get the lengths of both
    // Linked list.
    int len1 = getLength(l1);
    int len2 = getLength(l2);

    Node *lNode = NULL, *sNode = NULL;

    Node* temp1 = l1;
    Node* temp2 = l2;

    // If lengths differ, calculate the smaller Node
    // and padd zeros for smaller Node and ensure both
    // larger Node and smaller Node has equal length.
    if (len1 != len2)
    {
        lNode = len1 > len2 ? l1 : l2;
        sNode = len1 > len2 ? l2 : l1;
        sNode = paddZeros(sNode, abs(len1 - len2));
    }

    else
    {
        // If both list lengths are equal, then calculate
        // the larger and smaller list. If 5-6-7 & 5-6-8
        // are linked list, then walk through linked list
        // at last Node as 7 < 8, larger Node is 5-6-8
        // and smaller Node is 5-6-7.
        while (l1 && l2)
        {
            if (l1->data != l2->data)
            {
                lNode = l1->data > l2->data ? temp1 : temp2;
                sNode = l1->data > l2->data ? temp2 : temp1;
                break;
            }
            l1 = l1->next;
            l2 = l2->next;
        }
    }

    // After calculating larger and smaller Node, call
    // subtractLinkedListHelper which returns the subtracted
    // linked list.
    bool borrow = false;
    return subtractLinkedListHelper(lNode, sNode, borrow);
}

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

// Driver program to test above functions
int main()
{
    Node* head1 = newNode(1);
    head1->next = newNode(0);
    head1->next->next = newNode(0);

    Node* head2 = newNode(1);

    Node* result = subtractLinkedList(head1, head2);

    printList(result);

    return 0;
}

Output :

0 9 9

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