LeetCode206-反转链表

维护三个指针

public ListNode reverseList(ListNode head) {
        ListNode cur = head;
        ListNode pre = null;
        while(cur!=null){
            ListNode next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }
        return pre;
    }

递归实现

public ListNode reverseList2(ListNode head) {
        if (head==null || head.next==null){
            return head;
        }
        ListNode res = reverseList2(head.next);
        head.next.next = head;
        head.next = null;
        return res;
    }