Leetcode#24 Swap Nodes in Pairs

class Solution {
    public:
        ListNode* swapPairs(ListNode* head) {
            ListNode* pre_node = NULL;
            ListNode* slow = head;
            ListNode* fast = head;


            while(fast != NULL && fast->next != NULL) {
                fast = fast->next;
                /* exchange */

                if (pre_node != NULL) {
                    pre_node->next = fast;
                } else {
                    head = fast;
                }

                slow->next = fast->next;
                fast->next = slow;
                pre_node = slow; 

                /* for next iteration */
                fast = slow->next;
                slow = slow->next;
            }
            return head;
        }
};