反转链表java代码

/**
 * 反转链表
 * @Author sannian
 * @Date 2022/5/16 13:41
 * @Version 1.0
 */

public class LinkedListReversal {
    static class ListNode{
        int val;
        ListNode next;

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

    public static void main(String[] args) {
        ListNode node5 = new ListNode(5, null);
        ListNode node4 = new ListNode(4, node5);
        ListNode node3 = new ListNode(3, node4);
        ListNode node2 = new ListNode(2, node3);
        ListNode node1 = new ListNode(1, node2);
        ListNode recursion = recursion(node1);
        System.out.println(recursion);
    }

    /**
     * 迭代 重点在于两次标记
     * @param head
     * @return
     */
    public static ListNode iterate(ListNode head){
        ListNode prev = null,next;//循环的下一个节点标记
        ListNode curr = head;
        while (curr!=null){
            next = curr.next; // 把当前节点(如1)的下一个节点(如2)标记起来做循环条件
            curr.next = prev; // 把当前节点(如1)的前一个节点(如1的前一个节点是null)变成反转后的下一个节点
            prev = curr;      // 把当前节点(如1)标记成下一个节点(如2)的上一个节点
            curr = next;      // 把标记的要循环的下一个节点赋值给curr，循环就从2开始
        }
        return prev;
    }

    /**
     * 递归 从尾到头遍历
     */
    public static ListNode recursion(ListNode head){
        if (head ==null || head.next ==null){
            return head;
        }
        ListNode recursion = recursion(head.next);
        head.next.next = head;
        head.next = null;
        return recursion;
    }
}