【剑指offer】反转链表

public class  {
    int val;
    ListNode next = null;

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

public ListNode ReverseList(ListNode head) {
    ListNode current = head;
    ListNode previous = null;
    ListNode tail = null;

    while (current != null){
        ListNode next = current.next;

        if (next == null)
            tail = current;

        current.next = previous;
        previous = current;
        current = next;
    }

    return tail;
}