24. Swap Nodes in Pairs

Given a linked list, swap every two adjacent nodes and return its head.

For example,
Given 1->2->3->4, you should return the list as 2->1->4->3.

Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        
        if(head == NULL || head->next == NULL){
            return head;
        }
        ListNode* newHead = new ListNode(-1);
        ListNode* tail = newHead;
        newHead->next = head;
        
        while(tail && tail->next && tail->next->next){
            ListNode*  n = tail->next;
            ListNode*  nn = tail->next->next;
            
            //tail->next = nn;
            n->next = nn->next;
            nn->next = n;
            tail->next = nn;
            
            tail = tail->next->next;
        }
        
        return newHead->next;
    }
};