LeetCode - Reverse Linked List

ReverseLinkedList.java

//
    public ListNode newhead = null;
    public ListNode reverseList(ListNode head) {
        if(head == null) return head;
        helper(head, head.next);
        return newhead;
    }
    
    public void helper(ListNode pre, ListNode cur){
        if(cur == null){
            newhead = pre;
            return;
        }
        helper(cur, cur.next);
        cur.next = pre;
        pre.next = null;
    }

// recursive solution
public ListNode newhead = null;
public ListNode reverseList(ListNode head) {
        if(head == null) return head;
        helper(head);
        return newhead;
    }
    
    public ListNode helper(ListNode pre){
        if(pre.next == null){
            newhead = pre;
            return pre;
        }
        ListNode cur = helper(pre.next);
        cur.next = pre;
        pre.next = null;
        return pre;
    }
    
// iterative soltuion
public ListNode reverseList(ListNode head) {
        if(head == null) return head;
        ListNode dummyNode = new ListNode(0);
        ListNode pre = dummyNode;
        while(head != null){
            ListNode next = head.next;
            head.next = pre.next;
            pre.next = head;
            head = next;
        }
        return dummyNode.next;
    }