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.

Swap Nodes in Pairs.cpp

/**
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        ListNode tmpHead(-1);
        tmpHead.next = head;
        
        ListNode* pre = NULL;
        ListNode* post = &tmpHead;
        ListNode* result = NULL;
        
        if(head != NULL && head->next != NULL) {
            result = head->next;
        } else {
            result = head;
        }
        
        
        while(true) {
            
            pre = post;
            
            if(post->next != NULL) {
                post= post->next;
            } else {
                break;
            }
            
            if(post->next != NULL) {
                post= post->next;
            } else {
                break;
            }
            
            
            ListNode* first = pre->next;
            ListNode* second = first->next;
            
            ListNode* tmp = second->next;
            second->next = first;
            pre->next = second;
            first->next = tmp;
            
            
            post = post->next;
        }
        
        
        return result;
    }
};