反转链表

package com.algorithm;

//输入一个链表，反转链表后，输出链表的所有元素。
public class ReverseListMe {
    
    public ListNode ReverseList(ListNode head) {
        //运行时间：36ms 占用内存：528k
        if(head == null) {
            return head;
        }
        if(head.next == null) {
            System.out.println(head.val);
            return head;
        }
         ListNode front = head;//前一个
         ListNode back = head.next;//后一个
         ListNode guard = back.next;//第三个
         front.next = null;//将最后一个赋值为null
         while(back != null) {
             back.next = front;//把第二项的next 赋值为front
             front = back;
             back = guard;//将guard 赋值给front
             if(guard != null)
              guard = guard.next;//将guade的next赋值给guard
         }
         head = front;
         while(front != null) {
             System.out.println(front.val);
             front = front.next;
         }
         return head;
    }
    public static void main(String[] args) {
         ListNode la = new ListNode(1);
          ListNode lb = new ListNode(2);
          ListNode lc = new ListNode(3);
          la.next = lb;
          lb.next = lc;
          lc.next = null;
          new ReverseListMe().ReverseList(la);
    }
    /*class ListNode {
        int val;
        ListNode next = null;
        ListNode(int val) {
            this.val = val;
        }
    }*/
}

ListNode pre = null;   
ListNode next = null;    
while (head != null) {        
    next = head.next;        
    head.next = pre;        
    pre = head;        
    head = next;    
}    
    return pre;