剑指offer15-反转链表

输入一个链表，反转链表后，输出新链表的表头。

思路：pre指向第一个节点，current指向下个节点，while current不是空，保存current的下个节点，令current->next=pre;pre=current;current=current->next;

    ListNode* ReverseList(ListNode* pHead) {
        ListNode *pre,*nextNode,*current;
        if(pHead==NULL||pHead->next==NULL) return pHead;
        pre=pHead;
        current=pHead->next;
        pHead->next=NULL;
        while(current!=NULL)
        {
            nextNode=current->next;
            current->next=pre;
            pre=current;
            current=nextNode;
        }
        //current->next=pre;
        //Rhead=current;
        return pre;
    }