24. 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.

在不新建一个链表的情况下，交换链表相邻节点。

交换相邻节点A->B，A是head. 

ListNode* p = head;
head = head->next;
p->next = p->next->next;
head->next = p;

整体代码：

class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        if (!head || !head->next)
            return head;
        ListNode* p = head;
        head = head->next;
        p->next = p->next->next;
        head->next = p;
        ListNode* q = p->next;
        while (q && q->next) {
            ListNode *temp = q;
            q = q->next;
            temp->next = temp->next->next;
            q->next = temp;
            p->next = q;
            p = q->next;
            q = p->next;
        }
        return head;
    }
};