题目
定义一个函数,输入一个链表的头节点,反转该链表并输出反转后链表的头节点。
示例:
输入: 1->2->3->4->5->NULL
输出: 5->4->3->2->1->NULL
限制:
0 <= 节点个数 <= 5000
解题思路
详细思路请参考 206.反转链表
代码
解法一:迭代
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
class Solution {
public ListNode reverseList(ListNode head) {
ListNode pre = null;
ListNode cur = head;
while(cur != null){
ListNode temp = cur.next;
cur.next = pre;
pre = cur;
cur = temp;
}
return pre;
}
}
解法二:递归
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
class Solution {
public ListNode reverseList(ListNode head) {
if(head == null || head.next == null){
return head;
}
ListNode p = reverseList(head.next);
// 第一次 head = 4, head.next = 5
head.next.next = head;
head.next = null;
return p;
}
}