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/*
新建一个pre head node记录开头
*/

class Solution {
    public ListNode reverseList(ListNode head) {
        if(head == null || head.next == null)   return head;
        ListNode newhead = new ListNode(0);
        newhead.next = head;
        ListNode pre = newhead;
        ListNode cur = head;
        
        while(cur.next != null){
            ListNode next = cur.next;
            cur.next = next.next;
            next.next = pre.next;
            pre.next =next;
        }
        return newhead.next;
    }
}

/*
1->2->3->4->null
直接指向null
   2  ->3->4-null
  cur
   1  ->null
  pre
*/

class Solution {
    public ListNode reverseList(ListNode head) {
        ListNode prev = null;
        ListNode cur = head;
        
        while(cur != null) {
            ListNode nextNode = cur.next;
            cur.next = prev;
            prev = cur;
            cur = nextNode;
        }
        
        return prev;
    }
}