LeetCode:Reorder List C++与Java实现

LeetCode:Reorder List

题目（题目链接）：

Given a singly linked list L: L0→L1→…→Ln-1→Ln,
reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…

You must do this in-place without altering the nodes' values.

For example,
Given {1,2,3,4}, reorder it to {1,4,2,3}.

分析：先用快慢指针找到链表的中点，然后翻转链表后半部分，再和前半部分组合。需要注意的是把链表分成两半时，前半段的尾节点要置为NULL，翻转链表时也要把尾节点置为NULL。代码如下：

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; */class Solution {public:    void reorderList(ListNode *head) {        // IMPORTANT: Please reset any member data you declared, as        // the same Solution instance will be reused for each test case.        if(head == NULL || head->next == NULL)return;        ListNode *fastp = head, *lowp = head, *tail = NULL;        while(fastp != NULL && fastp->next != NULL)        {//利用快慢指针找到链表的中点            tail = lowp;            fastp = fastp->next->next;            lowp = lowp->next;        }        tail->next = NULL; //此时tail 指向前半段的结尾        reverseList(lowp);//翻转链表后半段        fastp = head;        tail = NULL;        while(fastp != NULL)        {            ListNode *tmp = lowp->next;            lowp->next = fastp->next;            fastp->next = lowp;            tail = lowp;            fastp = lowp->next;            lowp = tmp;        }        if(lowp != NULL)            tail->next = lowp;    }    void reverseList(ListNode* &head)    {//翻转链表        if(head == NULL || head->next == NULL)return;        ListNode *pre = head, *p = pre->next;        while(p != NULL)        {            ListNode *tmp = p->next;            p->next = pre;            pre = p;            p = tmp;        }        head->next = NULL;        head = pre;    }};

网上一般都是C++实现，自己做了个java版本，算法不同，网上也有同样算法的C++版。

代码如下：

/** * Definition for singly-linked list. * class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { *         val = x; *         next = null; *     } * } */import java.util.*;public class Solution {    public void reorderList(ListNode head) {        // IMPORTANT: Please reset any member data you declared, as        // the same Solution instance will be reused for each test case.        if(head==null||head.next==null||head.next.next==null) return;        Vector<ListNode> vin=new Vector<ListNode>();        Vector<ListNode> vout=new Vector<ListNode>();        ListNode p=head;        while(p!=null)        {            vin.add(p);            p=p.next;        }        int i=0;        int n=vin.size();        int k=n-1;        for(;k>n/2;i++,k--)        {            vout.add(vin.get(i));            vout.add(vin.get(k));        }        while(i<n/2+1)        {            vout.add(vin.get(i));            i++;        }        int j=0;        for(;j<vout.size()-1;j++)        {            vout.get(j).next=vout.get(j+1);        }        vout.get(j).next=null;            }}