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struct ListNode {	int val;	ListNode *next;	ListNode(int x) : val(x), next(NULL) {}};class Solution {//when finished the code, find come cases to test it firstpublic:	ListNode *swapPairs(ListNode *head) {		// Start typing your C/C++ solution below		// DO NOT write int main() function		ListNode* prev = NULL;		ListNode* first = NULL;		ListNode* second = head;		int k = 1;		while (second != NULL)		{			if (k%2 == 0)			{				first->next = second->next;				second->next = first;				if(prev != NULL)//when process prev pointer, we should be very careful					prev->next = second;				else head = second;				ListNode* tmp = first;				first = second;				second = tmp;			}			prev = first;			first = second;			second = second->next;			k++;		}		return head;	}};

second time

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; */class Solution {public:    void reverseList(ListNode* head, int k, ListNode*& reverseHead, ListNode*& reverseTail, ListNode*& nextHead)    {        reverseTail = head;        ListNode* prev = head;        ListNode* cur = head->next;        for(int i = 0; i < k-1; ++i)        {            if(cur == NULL) break;            ListNode* tmp = cur->next;            cur->next = prev;            prev = cur;            cur = tmp;        }        reverseHead = prev;        nextHead = cur;    }    ListNode *swapPairs(ListNode *head) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        ListNode dummy(-1);        ListNode* prev = &dummy;                ListNode* nextHead = head;        while(nextHead != NULL)        {            ListNode* reverseHead;            ListNode* reverseTail;            reverseList(nextHead, 2, reverseHead, reverseTail, nextHead);            prev->next = reverseHead;            prev = reverseTail;        }        prev->next = NULL;        return dummy.next;    }};

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