Merging two linked list into one

[sgk]

I have created this program to merge two linked list into one ,but however everything is working fine but on execution it providing a segmentation fault error ( on calling merging function )

code :


#include<iostream>
#include<stdio.h>
using namespace std;

struct node{
int data;
node* link;
};

typedef node* nodeptr;

nodeptr head_first=NULL;

nodeptr head_second=NULL;

bool insert_first(int n,nodeptr &head_first)
{

nodeptr newnode = new node;

if(!newnode) return false;

newnode->data=n;

newnode->link=head_first;

head_first=newnode;

return true;

}


bool insert_second(int n,nodeptr &head_second)
{

nodeptr newnode = new node;

if(!newnode) return false;

newnode->data=n;

newnode->link=head_second;

head_second=newnode;

return true;

}


void show_first(nodeptr &head_first){

nodeptr current=new node;

current=head_first;

while(current!=NULL)
{
cout<<current->data<<" -> ";
current=current->link;
}

}

void show_second(nodeptr &head_second){

nodeptr current=new node;

current=head_second;

while(current!=NULL)
{
cout<<current->data<<" -> ";
current=current->link;
}

}


nodeptr merge_two_list(nodeptr &head_first,nodeptr &head_second)
{

nodeptr result=NULL;

if(head_first==NULL) return head_second;
else if(head_second=NULL) return head_first;


if (head_first->data <= head_second->data)
{
result = head_first;
result->link = merge_two_list(head_first->link, head_second);
}
else
{
result = head_second;
result->link = merge_two_list(head_first, head_second->link);
}
return result;
}


int main()
{

insert_first(1,head_first); //inserting into first list
insert_first(2,head_first);
insert_first(3,head_first);

insert_second(4,head_second); //inserting into second list
insert_second(5,head_second);
insert_second(6,head_second);

cout<<"List 1 :"<<endl;

show_first(head_first); //showing first list

cout<<endl;

cout<<"List 2 :"<<endl;

show_second(head_second); //showing second list

cout<<endl;

nodeptr final =new node;

final=merge_two_list(head_first,head_second); //storing result into final

while(final!=NULL) //walking from final to null
{

cout<<final->data<<" -> ";
final=final->link;

}

cout<<endl;

return 0;

}


o/p :

List 1 :
3 -> 2 -> 1 ->
List 2 :
6 -> 5 -> 4 ->
Segmentation fault (core dumped)



thanks in advance :)

[ne555]

http://www.cplusplus.com/forum/general/112111/

foo.cpp: In function ‘node* merge_two_list(node*&, node*&)’:
foo.cpp:56:26: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
	else if(head_second=NULL) return head_first;