LeetCode--反转链表

206.反转链表

给你单链表的头节点 head ，请你反转链表，并返回反转后的链表。

示例1

输入：head = [1,2,3,4,5]
输出：[5,4,3,2,1]

示例2

输入：head = [1,2]
输出：[2,1]

示例3

输入：head = []
输出：[]

Solution

public class ListNode {


    int val;
    ListNode next;

    ListNode() {
    }

    ListNode(int val) {
        this.val = val;
    }

    ListNode(int val, ListNode next) {
        this.val = val;
        this.next = next;
    }


    public static void main(String[] args) {

        ListNode head = new ListNode(1, new ListNode(2, new ListNode(3)));

        ListNode reverseList = reverseList(head);
        while (Objects.nonNull(reverseList)) {
            System.out.println(reverseList.val);
            reverseList = reverseList.next;
        }
    }

    /**
     * 迭代方式
     *
     * @param head
     * @return
     */
    public static ListNode reverseList(ListNode head) {
        if (head == null) {
            return null;
        }

        ListNode prev = null;
        while (head != null) {
            ListNode next =  head.next;
            head.next = prev;
            prev = head;
            head = next;
        }
        return prev;
    }
}