Linked List - 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.

这题要求遍历链表，两两交换，也算是一道比较简单的题目，我们只需要拿到需要交换的前驱节点就可以了。直接上代码：

class Solution {
public:
    ListNode *swapPairs(ListNode *head) {
        if(!head || !head->next) {
            return head;
        }
        ListNode dummy(0);
        ListNode* p = &dummy;
        dummy.next = head;
        while(p && p->next && p->next->next) {
            ListNode* n = p->next;
            ListNode* nn = p->next->next;
            p->next = nn;
            n->next = nn->next;
            nn->next = n;
            p = p->next->next;
        }
        return dummy.next;
    }
};