package LeetCode;

public class LeetCode206 {

// https://leetcode.com/problems/reverse-linked-list/description/

// 时间复杂度: O(n)

// 空间复杂度: O(1)

// Definition for singly-linked list.

public class ListNode {

int val;

ListNode next;

ListNode(int x) { val = x; }

}

public ListNode reverseList(ListNode head) {

ListNode pre = null;

ListNode cur = head;

while(cur != null){

ListNode next = cur.next;

cur.next = pre;

pre = cur;

cur = next;

}

return pre;

}

// 递归方式

public ListNode reverseList2(ListNode head) {

// 递归终止条件

if(head == null|| head.next == null)

return head;

// head==1 --->head==2 return head ==2

ListNode rhead = reverseList2(head.next);

// head->next此刻指向head后面的链表的尾节点

// head->next->next = head把head节点放在了尾部

// TODO 最后一个节点指向倒数第二个节点

head.next.next = head;

// 最后一个节点指向null

head.next = null;

return rhead;

}

}