题解 | #反转链表#

    ListNode* ReverseList(ListNode* head) {
        // write code here
        if(head == NULL) return NULL;
        ListNode* cur = head;
        ListNode* pre = NULL;
        while(cur != NULL){
            ListNode* temp = cur->next;
            cur->next = pre;
            pre = cur;
            cur = temp;
        }
        return pre;
    }
递归
    ListNode* ReverseList(ListNode* head) {
        if(head == NULL || head -> next == NULL)
            return head;
        ListNode* newHead = ReverseList(head->next);
        // 假如有ABCD head = a
        // B->next = A 相当于B指向了A
        head->next->next = head;
        // 再将A->next 指向null
        head->next = NULL;
        return newHead;
    }