public static class ListNode {
        int val;
        ListNode next;

        ListNode() {
        }

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

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

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。

public static void main(String [] args){
  public static ListNode reverse(ListNode head){
      ListNode pre = null;
      ListNode current = head;
      while(current != null){
        ListNode next =current.next;
        current.next = pre;
        pre = current;
        current = next;
    }  

  return pre;
 }
}