剑指offer_15

public class ListNode {
    int val;
    ListNode next = null;
    ListNode(int val) {
        this.val = val;
    }
}*/
public class  {
    public ListNode ReverseList(ListNode head) {
		if(head == null || head.next == null){
            return head;
        }
        
        ListNode pre = null;
        ListNode cur = head;
        ListNode post = head.next;
        
        while(post != null){
            cur.next = pre;
            pre = cur;
            cur = post;
            post = post.next;
        }
        //循环退出条件post == null
        //最后一个节点还未指向它的前一节点
        cur.next = pre;
        return cur;
    }
}