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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;
}
}