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.

题目大意：

ListNode* swapPairs(ListNode* head) {
        ListNode *p1 = head, *p2;
        int tmp;
        
        while(p1) {
            p2 = p1->next;
            if (p2) {
                tmp = p1->val;
                p1->val = p2->val;
                p2->val = tmp;
                p1 = p1->next;
            }
            p1 = p1->next;
        }
        
        return head;
 }

换指针：

ListNode* swapPairs(ListNode* head) {
        ListNode *p1 = head, *p2, *tmp;
        
        if (p1 && p1->next) {
            head = head->next;
        }
        
        tmp = head;
        
        while(p1) {
            p2 = p1->next;
            if (p2) {
                p1->next = p2->next;
                p2->next = p1;
                if (tmp != head)
                    tmp ->next = p2;
                tmp = p1;
            }
            p1 = p1->next;
        }
        
        return head;
}

ListNode* swapPairs(ListNode* head) {
        ListNode* p1;       
        if(head && head->next){  
            p1 = head->next; 
            head->next = swapPairs(head->next->next);
            p1->next = head;
            head = p1; 
       }
       return head;
}