Java实现反转单链表

反转单链表是一个常见的编程问题，可以使用迭代或递归的方式来实现。下面分别给出这两种方式的Java代码实现：

使用迭代方式实现反转单链表：

class ListNode {
    int val;
    ListNode next;

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

public class ReverseLinkedList {

    public ListNode reverseList(ListNode head) {
        ListNode prev = null;
        ListNode curr = head;

        while (curr != null) {
            ListNode nextTemp = curr.next;
            curr.next = prev;
            prev = curr;
            curr = nextTemp;
        }

        return prev;
    }

    public static void main(String[] args) {
        // 创建链表 1 -> 2 -> 3 -> 4 -> 5
        ListNode head = new ListNode(1);
        head.next = new ListNode(2);
        head.next.next = new ListNode(3);
        head.next.next.next = new ListNode(4);
        head.next.next.next.next = new ListNode(5);

        // 创建 ReverseLinkedList 对象
        ReverseLinkedList reverseLinkedList = new ReverseLinkedList();

        // 反转链表
        ListNode reversedHead = reverseLinkedList.reverseList(head);

        // 输出反转后的链表 5 -> 4 -> 3 -> 2 -> 1
        while (reversedHead != null) {
            System.out.print(reversedHead.val + " ");
            reversedHead = reversedHead.next;
        }
    }
}

使用递归方式实现反转单链表：

class ListNode {
    int val;
    ListNode next;

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

public class ReverseLinkedList {

    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;
    }

    public static void main(String[] args) {
        // 创建链表 1 -> 2 -> 3 -> 4 -> 5
        ListNode head = new ListNode(1);
        head.next = new ListNode(2);
        head.next.next = new ListNode(3);
        head.next.next.next = new ListNode(4);
        head.next.next.next.next = new ListNode(5);

        // 创建 ReverseLinkedList 对象
        ReverseLinkedList reverseLinkedList = new ReverseLinkedList();

        // 反转链表
        ListNode reversedHead = reverseLinkedList.reverseList(head);

        // 输出反转后的链表 5 -> 4 -> 3 -> 2 -> 1
        while (reversedHead != null) {
            System.out.print(reversedHead.val + " ");
            reversedHead = reversedHead.next;
        }
    }
}

无论是迭代还是递归，反转单链表的思路都是将当前节点的指针指向前一个节点，从而实现链表的反转。以上两种方式都可以正确实现单链表的反转。