Swap the nth and the mth node in a linked list

In this case n = 2 and m = 4.
Please give a linked list of size > 4. Corner cases are not handled. Only the main swapping part is handled.

 

#include <iostream>
#include <algorithm>
#include <cstdlib>

using namespace std;

typedef struct node{
 int data;
 node *next;
};

node *allocate(){
 node *ptr = (node *)malloc(sizeof(node));
 ptr -> next = NULL;
 return ptr;
}

node *f(){
 int x;
 cin >> x;
 if(x == -1)
 return NULL;
 node *head = allocate();
 head ->data = x;
 head ->next = f();
 return head;
}

int main()
{
 cout << "Enter items of list:-\nType -1 to exit.\n";
 node *head;
 head = NULL;
 head = f();
 node *temp = head;
 while(temp!= NULL)
 cout << temp->data << " ", temp = temp->next;
 node *ptr2, *ptr4;
 temp = head;
 for(int i = 0; i < 4; i++){
 if(i == 0)
 ptr2 = temp;
 else if(i == 2)
 ptr4 = temp;
 temp = temp -> next;
 }

// cout << ptr2->data << " " << ptr4->data << endl;
 temp = (ptr2->next) -> next;
 ptr2->next->next = ptr4->next->next;
 ptr4->next->next = temp;

 temp = ptr2->next;
 ptr2->next = ptr4->next;
 ptr4->next = temp;
 cout << endl;
 temp = head;
// system ("pause");
 while(temp!= NULL)
 cout << temp->data << " ", temp = temp->next;

 return 0;
}