Why am I getting segmentation fault while implementing linked list? - c++

Question

In this function, I get segmentation fault. I think it has something to do with memory allocation. What mistake am I making?
Now, if I initialize Node* a =NULL, i get my head pointer as NULL in the end.
struct Node {
int data;
struct Node* next;
Node(int x) {
data = x;
next = NULL;
}
};
Node* addTwoLists(Node* first, Node* second) {
// Code here
Node *a;
Node *head = a;
int bor = 0;
while(first->next && second->next) {
int ans = first->data + second->data;
a = new Node((ans%10)+bor);
bor = ans/10;
a=a->next;
first = first->next;
second = second->next;
}
return head;
}

Answer 1

a is uninitialized. You must not use a until you assign a value
you never again assign to head, so it could never be anything else.

Answer 2

It's not the allocation, it's the pointer use that is all wrong.
Here's how it should look. This code maintains a variable last which is the last node added to the list. You need this variable so you can at the end of the list. You were obviously trying to do this yourself, but got the logic wrong.
Node* addTwoLists(Node* first, Node* second) {
Node *last = NULL;
Node *head = NULL;
int bor = 0;
while(first->next && second->next) {
int ans = first->data + second->data;
Node* a = new Node((ans%10)+bor);
if (head == NULL) {
head = last = a; // first node, update head and end of list
}
else {
last->next = a; // add a to the end of the list
last = a; // update the end of the list
}
bor = ans/10;
first = first->next;
second = second->next;
}
return head;
}
Untested code.

Answer 3

For starters the variable head has indeterminate value and is not changed in the function.
Node *a;
Node *head = a;
Changing the variable a does not mean changing of the value of the expression a->next.
// ...
a = new Node((ans%10)+bor);
//...
a=a->next;
The function can be written the following way (without testing)
Node * addTwoLists( const Node *first, const Node *second )
{
const int Base = 10;
Node *head = nullptr;
int bor = 0;
Node **current = &head;
for ( ; first != nullptr && second != nullptr; first = first->next, second = second->next )
{
int sum = first->data + second->data + bor;
*current = new Node( sum % Base );
bor = sum / Base;
current = &( *current )->next;
}
if ( bor )
{
*current = new Node( bor );
}
return head;
}
Here is a demonstrative program
#include <iostream>
struct Node
{
explicit Node( int data, Node *next = nullptr ) : data( data ), next( next )
{
}
int data;
Node *next;
};
void push_front( Node **head, int x )
{
*head = new Node( x, *head );
}
Node * addTwoLists( const Node *first, const Node *second )
{
const int Base = 10;
Node *head = nullptr;
int bor = 0;
Node **current = &head;
for ( ; first != nullptr && second != nullptr; first = first->next, second = second->next )
{
int sum = first->data + second->data + bor;
*current = new Node( sum % Base );
bor = sum / Base;
current = &( *current )->next;
}
if ( bor )
{
*current = new Node( bor );
}
return head;
}
std::ostream & display_list( const Node *head, std::ostream &os = std::cout )
{
for ( ; head != nullptr; head = head->next )
{
os << head->data << ' ';
}
return os;
}
int main()
{
const int N = 10;
Node *list1 = nullptr;
Node *list2 = nullptr;
for ( int i = 1; i < N; i++ ) push_front( &list1, i );
for ( int i = N; --i != 0; ) push_front( &list2, i );
display_list( list1 ) << '\n';
display_list( list2 ) << '\n';
Node *list3 = addTwoLists( list1, list2 );
display_list( list3 ) << '\n';
}
Its output is
9 8 7 6 5 4 3 2 1
1 2 3 4 5 6 7 8 9
0 1 1 1 1 1 1 1 1 1

Answer 4

You may get segmentation fault for various reasons here.
If first or second is NULL then you will get segmentation fault. So make sure that if these two nodes are not NULL.
You didn't initialize a. So initialize it first.
And as you want head variable should contain starting node of the answer list so you need to assign node whenever you get start of the list.
Just add this line after a = new Node((ans%10)+bor);
if(head == NULL) head = a;