第7讲5*

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if (!head || !head->next) return head;

        auto p = head, q = p->next;
        while(q)
        {
            auto o = q->next;
            q->next = p;  //反转
            p = q, q = o; //下两个
        }
        head->next = NULL;
        return p;
    }
};

// 1 -----> 2 -----> 3
// p        q        o
// 1 <----- 2 -----> 3 -----> 4
//          p        q        o