Reverse Second half of LinkedList Java实现

第三道Amazon面试coding题练习。

这道题是说，有一个linkedlist  {1,2,3,4,5,6,7,8}转换为{1,2,3,4,8,7,6,5}，初看不难，其实也不难。LinkedList 非常好理解，就是多一个指针。

翻转一串LinkedList的示例网上大把，随便看看就好。

package quinn;

import quinn.List.ListNode;

public class ReverseLinkedList {

public static void main(String[] args) {
// TODO Auto-generated method stub
int[] list = {1,2,3,4,5,6,7,8,9};

//////////////////////////////////////////////////////////////////
///创建linkedList
//////////////////////////////////////////////////////////////////
ListNode head = new ListNode(list[0]);
ListNode node = head;
for(int i=1; i < list.length; i++)
{
ListNode NewNode = new ListNode(list[i]);
node.next = NewNode;
node = NewNode;
}

int length = getLength(head);//假装不知道长度是多少，没有看到具体的题，就让我随便写写吧

head = ReverseHalf(length,head);//先取一半，再翻转

OutPutList(head);

}

private static ListNode ReverseHalf(int length, ListNode head) {
// TODO Auto-generated method stub
ListNode first = head;

ListNode halfhead;
if(length<4)//特殊情况，如果小于4了，完全没有必要翻转
return head;
else
{
int halflen = (int) Math.ceil((double)length/2);//没看具体题目，假定9/2的时候是5
System.out.println("一半的长度是:\t"+halflen);

for(int i=0; i < halflen-1;i++)//这里减一是为了让指针停留在前一半最后一个节点上
{
head = head.next;
}

halfhead = RecursiveReverse(head.next);//翻转后一半

head.next = halfhead;//前一半连接上翻转后的后一半

}
return first;
}

private static int getLength(ListNode head) {
// TODO Auto-generated method stub
int size = 0;
while(head!= null)
{
size++;
head = head.next;
}
return size;
}

private static ListNode RecursiveReverse(ListNode head)
{
//递归的进行反转，递归的比较好理解
if(head==null || head.next==null)
{
return head;
}

ListNode second = head.next;
head.next = null;

ListNode rest = RecursiveReverse(second);
second.next = head;
return rest;
}

private static ListNode reverseList(ListNode head) {
// TODO Auto-generated method stub
if(head==null||head.next==null)
return head;

ListNode p1 = head;
ListNode p2 =head.next;

head.next = null;
while(p1!=null && p2!=null)
{
ListNode t = p2.next;
p2.next = p1;
p1 = p2;
if(t!=null)
{
p2 = t;
}else
break;
}
return p2;
}

private static void OutPutList(ListNode head) {
// TODO Auto-generated method stub
while(head!=null)
{
System.out.println(head.val);
head =head.next;
}
}

}

LinkedList是一个数据结构，在这里，我定义如下：

package quinn;

public class List {

public static class ListNode
{
int val;
ListNode next;

public ListNode(int val)
{
this.val = val;
this.next = null;
}

}

}