206.反转链表

206.反转链表

方法一： 双指针

class Solution {
    public ListNode reverseList(ListNode head) {
        ListNode pre = null;
        ListNode cur = head;
        ListNode temp = null;
        while(cur != null){
            temp = cur.next;//保存下一个节点
            cur.next = pre;
            pre = cur;
            cur = temp;
        }
        return pre;
    }
}

方法二：递归

class Solution {
    public ListNode reverseList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        ListNode newHead = reverseList(head.next);
        head.next.next = head;
        head.next = null;
        return newHead;
    }
}