/**

* Swap Nodes in Pairs

* Dummy head.

* cpselvis (cpselvis@gmail.com)

* August 22th, 2016

*/

#include<iostream>

using namespace std;

struct ListNode {

int val;

ListNode *next;

ListNode(int x) : val(x), next(NULL) {}

};

class Solution {

public:

ListNode* swapPairs(ListNode* head) {

ListNode *dummy = new ListNode(-1);

dummy -> next = head;

ListNode *prev = dummy;

while (head && head -> next)

{

ListNode *next = head -> next -> next;

prev -> next = head -> next;

prev -> next -> next = head;

head -> next = next;

prev = head;

head = next;

}

return dummy -> next;

}

};

int main(int argc, char **argv)

{

ListNode *l1 = new ListNode(1);

l1 -> next = new ListNode(2);

l1 -> next -> next = new ListNode(3);

l1 -> next -> next -> next = new ListNode(4);

Solution s;

ListNode *l2 = s.swapPairs(l1);

while (l2 != NULL)

{

cout << l2 -> val << endl;

l2 = l2 -> next;

}

}