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public class ????II {
    public class ListNode {
        int val;
        ListNode next;
        public ListNode(int val) {
            this.val = val;
        }
    }
    public ListNode rever(ListNode head) {
        if (head == null) {
            return null;
        }
        ListNode pre = null;
        ListNode curr = head;
        while (curr != null) {
            ListNode future = curr.next;
            curr.next = pre;
            pre = curr;
            curr = future;
        }
        return pre;
    }
    
    public static void main(String[] args) {
        // ????
        ListNode s1 = new ListNode(1);
        ListNode s2 = new ListNode(2);
        ListNode s3 = new ListNode(3);
        ListNode s4 = new ListNode(4);
        s1.next = s2;
        s2.next = s3;
        s3.next = s4;
        ListNode res = rever(s1);
        while (res != null) {
            System.out.println(res.val);
            res = res.next;
        }
    }
}

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public class ????II {
    public class ListNode {
        int val;
        ListNode next;
        public ListNode(int val) {
            this.val = val;
        }
    }
    public ListNode rever1(ListNode head) {
        if (head == null) {
            return null;
        }
        ListNode dumy = new ListNode(0);
        dumy.next = head;
        ListNode pre = dumy;
        ListNode curr = head;
        while (curr.next != null) {
            ListNode future = curr.next;
            curr.next = future.next;
            future.next = dumy.next;
            pre.next = future;
        }
        return dumy.next;
    }
    
    public static void main(String[] args) {
        // ????
        ListNode s1 = new ListNode(1);
        ListNode s2 = new ListNode(2);
        ListNode s3 = new ListNode(3);
        ListNode s4 = new ListNode(4);
        s1.next = s2;
        s2.next = s3;
        s3.next = s4;
        ListNode res = rever1(s1);
        while (res != null) {
            System.out.println(res.val);
            res = res.next;
        }
    }
}

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