单向链表的就地逆置

 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
void (ListNode* head){
    // head 是头结点 head->next 指向第一个结点
    ListNode *p = head->next;
    ListNode *q = head->next;
    while(p){
        ListNode *n = p->next;
        p->next = n->next;
        n->next = q;
        q = n;
    }
    head->next = q;
}