第十三周算法

题目：

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.

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解答：

ListNode* swapPairs(ListNode* head) {    ListNode **pp = &head, *a, *b;    while ((a = *pp) && (b = a->next)) {        a->next = b->next;        b->next = a;        *pp = b;        pp = &(a->next);    }    return head;}