public class ListNode{
    int val;
    ListNode next;
    ListNode(int val){
        this.val = val;
    }
}
//递归反转链表 时间复杂度为O(n); 空间复杂度为O(n);
public reverse(ListNode head){
    if(head == null || head.next == null){
        return head;
    }
    ListNode temp = head.next;
    ListNode newHead = reverse(temp);
    temp.next = head;
    head.next = null;
    return newHead;
}
//迭代反转链表，时间复杂度为O(n); 空间复杂度为O(1);
class Solution{
    public ListNode reverseList(ListNode head){
        ListNode prev = null;
        ListNode cur = head;
        while(cur != null){
            ListNode nexttemp = cur.next;
            cur.next = prev;
            prev = cur;
            cur = nexttemp;
        }
        return prev;
    }
}