Summing two linked lists where each node is a digit - c++

Question

I'm trying to write a function that sums integers, each of which are represented by a linked list where each node->data is a digit 0-9. The least significant digit is at the head of the list and the most is at the tail.
This is from the book Cracking the Coding Interview. Here is my code:
SinglyLinked<int>& addLists(SinglyLinked<int>& ll1, SinglyLinked<int>& ll2)
{
SinglyLinked<int> *sumList = new SinglyLinked<int>;
ListNode<int> *node1 = ll1.head; ListNode<int> *node2 = ll2.head;
int sum, carry = 0;
while(node1 || node2)
{
if(node1)
sum += node1->data;
if(node2)
sum += node2->data;
sum += carry;
carry = 0;
sumList->insertAtEnd(sum%10);
if(sum > 9)
carry = 1;
sum = 0;
node1 = node1->next; node2 = node2->next;
}
return *sumList;
}
First of all, is this code correct? It seems to work, but it seg faults when given two linked lists (integers) of different lengths. I'm wondering if the problem was only intended to solve for the case where the integers are of the same length. If not, how would I go about summing two lists of different lengths? My naive solution is to store the length of each list and then use that to create the digits which only one of the numbers will contribute to, until the two integers are aligned. Is there something more elegant than that?

Answer 1

It segfaults on different length lists, because then the node1 or node2 points to null.
Change
node1 = node1->next; node2 = node2->next;
to
if (node1)
node1 = node1->next;
if (node2)
node2 = node2->next;

Answer 2

while(node1 || node2)
If either node is ok keep going. But the block expects both nodes to be valid when it gets here:
node1 = node1->next; node2 = node2->next;
You needs your "nexts" in the if node1 checks:
if(node1) {
sum += node1->data;
node1 = node1->next;
}
etc.

Answer 3

There is one condition, where node1 and node2 will point to NULL and if there is a carry from the previous digit operation, it will not be appended to the end of the sumlist. For example, 6->5->4 + 4->5->6 should be 0->1->1->1 but sumlist will be 0->1->1. So before return line you should add:
if (carry)
sumList->insertAtEnd(carry);

Answer 4

Another solution to this problem is when you add the numbers in each list, add them together to get the big sum equal to the sum of two lists, convert that sum into a string, and append each character of the string to a new list. Hope it helps someone. Below is the code.
node.h file
#ifndef node_h
#define node_h
class LinkedList
{
private:
struct node
{
int data;
node *next;
};
node *head;
public:
LinkedList ();
node *createNode (int key);
void appendNodeBack (const int &key);
void printList ();
int SumOfNodes (const LinkedList list1);
};
#endif
node.cpp file
#include "node.h"
#include <math.h>
LinkedList::LinkedList ():head(NULL) {}
LinkedList::node *LinkedList::createNode (int key)
{
node *newNode = new node;
newNode->data = key;
newNode->next = NULL;
return newNode;
}
void LinkedList::appendNodeBack (const int &key)
{
node *newNode = createNode (key);
//if tree is empty
if (head == NULL)
{
head = newNode;
return;
}
//if tree is not empty
//traverse to the last node in the list
node *temp = head;
while (temp->next != NULL)
temp = temp->next;
temp->next = newNode;
}
void LinkedList::printList ()
{
//if tree is empty
if (head == NULL)
{
std::cout << "Tree is empty!\n";
return;
}
//if tree not empty
node *temp = head;
while (temp != NULL)
{
std::cout << temp->data<<"-->";
temp = temp->next;
}
std::cout << "NULL\n";
}
int LinkedList::SumOfNodes (const LinkedList list1)
{
//find the length of the list
node *temp = head;
int count = 0;
while (temp != NULL)
{
count++;
temp = temp->next;
}
//re-assign temp to head
temp = head;
//calculate the sum
unsigned int sum = 0;
for (unsigned int i = 1; i < pow (10, count); i = i * 10)
{
sum = sum + temp->data * i;
temp = temp->next;
}
return sum;
}
main.cpp file
#include <iostream>
#include "node.cpp"
int main ()
{
LinkedList list1, list2, list3;
list1.appendNodeBack (2);
list1.appendNodeBack (3);
list1.appendNodeBack (5);
list1.appendNodeBack (4);
list2.appendNodeBack (5);
list2.appendNodeBack (6);
list2.appendNodeBack (7);
list2.appendNodeBack (8);
list1.printList ();
std::cout << list1.SumOfNodes (list1) << std::endl;
list2.printList ();
std::cout << list2.SumOfNodes (list2) << std::endl;
unsigned int sum = list1.SumOfNodes (list1) + list2.SumOfNodes (list2);
//convert the number to string
std::string str = std::to_string (sum);
//append integer value to the new list
for (unsigned int i = 0; i < str.length (); i++)
list3.appendNodeBack (int (str[i] - '0'));
std::cout << "The new list becomes\n";
list3.printList();
return 0;
}