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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        ListNode* cur = head;
        ListNode* pre = head;
        ListNode* next = NULL;
        if (head == NULL) return head;
        if (head->next != NULL) {
            head = head->next;
            next = head->next;
            head->next = cur;
            cur->next = next;
            pre = cur;
            cur = next;
            if (cur) {
              next = cur->next;
            } else {
                next = NULL;
            }

        }
        while (next != NULL) {
            pre->next = cur->next;
            cur->next = next->next;
            next->next = cur;
            pre = cur;
            cur = cur->next;
            if (cur) {
              next = cur->next;
            } else {
                next = NULL;
            }
        }
        return head;
    }
};