最基本的手动实现反转链表 总是解决不了空指针 请问问题出在哪

public class ReverseList {
class ListNode {
ListNode next;
E val;

    public ListNode(E value) {
        val = value;
        next = null;
    }

    public ListNode(E value, ListNode<E> n) {
        val = value;
        next = n;
    }

    public void setNext(ListNode<E> n) {
        next = n;
    }
}

static ListNode reverseList(ListNode head){
    if(head==null){
        return null;
    }
    ListNode curNode = head;
    ListNode preNode = null;
    while(curNode!=null){
        ListNode nextNode = curNode.next;
        curNode.next=preNode;
        preNode = curNode;
        curNode = nextNode;
        System.out.println("This is preNode: "+preNode.val);
    }
    return preNode;
}

public static void main(String[] args) {
    ReverseList rl=  new ReverseList();
    ReverseList.ListNode head = rl.new ListNode(1);
    for (int i = 2; i <= 5; i++) {
        head.next = rl.new ListNode(i);
        head.next = head.next.next;

    }
    ListNode ln = reverseList(head);
    while(ln.next!=null) {
        System.out.println(ln.val);
        ln = ln.next;
    }
}

curNode.next;可能是空值，交换之前要先判断。

第36、37句赋值有问题，应该定义个临时变量赋值，37句又赋成空指针了