C++ Program To Subtract Two Numbers Represented As Linked Lists

// 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;
        }
    }
      // If both lNode and sNode still 
      // have NULL value, then this means 
      // that the  value of both of the 
      // given linked lists is the same 
      // and hence we can directly return 
      // a node with value 0.
    if(lNode==NULL&&sNode==NULL)
    {
      return newNode(0);
    }

    // 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("");
}

// Driver code
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;
}