class Solution {
    public ListNode reverseList(ListNode head) {
        if(head==null||head.next==null) return head;
        ListNode prev=null, curr=head, after=head.next;
        
        while(curr.next.next!=null){
            curr.next=prev;
            prev=curr;
            curr=after;
            after=after.next;
        }
        
        curr.next=prev;
        after.next=curr;
        
        return after;
    }
}