# 重排链表

<p>给定一个单链表 <code>L</code><em> </em>的头节点 <code>head</code> ，单链表 <code>L</code> 表示为：</p>

<p><code> L<sub>0 </sub>→ L<sub>1 </sub>→ … → L<sub>n-1 </sub>→ L<sub>n </sub></code><br />
请将其重新排列后变为：</p>

<p><code>L<sub>0 </sub>→ L<sub>n </sub>→ L<sub>1 </sub>→ L<sub>n-1 </sub>→ L<sub>2 </sub>→ L<sub>n-2 </sub>→ …</code></p>

<p>不能只是单纯的改变节点内部的值，而是需要实际的进行节点交换。</p>

<p> </p>

<p><strong>示例 1:</strong></p>

<p><img alt="" src="https://img-service.csdnimg.cn/img_convert/749247317f1c95079a7daa32d6b7da96.png" style="width: 240px; " /></p>

<pre>
<strong>输入: </strong>head = [1,2,3,4]
<strong>输出: </strong>[1,4,2,3]</pre>

<p><strong>示例 2:</strong></p>

<p><img alt="" src="https://img-service.csdnimg.cn/img_convert/f425cb8c2d72409c26564abcdb8cb629.png" style="width: 320px; " /></p>

<pre>
<strong>输入: </strong>head = [1,2,3,4,5]
<strong>输出: </strong>[1,5,2,4,3]</pre>

<p> </p>

<p><strong>提示：</strong></p>

<ul>
	<li>链表的长度范围为 <code>[1, 5 * 10<sup>4</sup>]</code></li>
	<li><code>1 <= node.val <= 1000</code></li>
</ul>


## template

```cpp
#include <bits/stdc++.h>
using namespace std;

struct ListNode
{
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};

class Solution
{
public:
    void reorderList(ListNode *head)
    {
        ListNode *l1 = head, *l2 = head;
        ListNode *tep = head;
        int len = 0;
        while (tep)
        {
            len++;
            tep = tep->next;
        }
        if (len <= 2)
            return;
        int len1 = len / 2 + len % 2;
        int len2 = len / 2;
        for (int i = 0; i < len1 - 1; i++)
        {
            head = head->next;
        }
        l2 = head->next;
        head->next = nullptr;
        head = l1;
        stack<ListNode *> stk;
        while (l2)
        {
            stk.push(l2);
            l2 = l2->next;
        }
        while (!stk.empty())
        {
            tep = stk.top();
            stk.pop();
            tep->next = l1->next;
            l1->next = tep;
            l1 = tep->next;
        }
    }
};


```

## 答案

```cpp

```

## 选项

### A

```cpp

```

### B

```cpp

```

### C

```cpp

```