要三个指针

class Solution {
public:
    ListNode* ReverseList(ListNode* pHead) {
        if(!pHead) return NULL;
        ListNode* pre=NULL;
        ListNode* cur=pHead;
        ListNode* tmp;
        while(cur->next){
            tmp=cur->next;
            cur->next=pre;
            pre=cur;
            cur=tmp;
        }
        cur->next=pre;
        return cur;
    }
};