Reorder List leetcode

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}.

链表操作的思路往往比较容易想，但是实现起来就非常复杂。

这里思路就是

1.将链表分为前后两部分

2.将后部分链表反转，这里反转我使用了一种新的方法（制造一个新的链表，将旧链表的结点依次添加到新链表的前端）

3.构造新的链表，前后两部分链表结点交替添加到新链表中

4.构造完成

void reorderList(ListNode* head) {
    if (head == nullptr || head->next == nullptr)
        return;
    // 分割前后两部分
    ListNode *slow = head;
    ListNode *fast = head;
    while (fast->next != nullptr && fast->next->next != nullptr)
    {
        slow = slow->next;
        fast = fast->next->next;
    }
    ListNode *head1 = head;
    ListNode *head2 = slow->next;
    slow->next = nullptr;
    head = head1;
    // 逆置链表
    ListNode *tail2 = head2;
    head2 = head2->next;
    tail2->next = nullptr;
    while (head2 != nullptr)
    {
        ListNode *temp = head2->next;
        head2->next = tail2;
        tail2 = head2;
        head2 = temp;        
    }
    // 交替构造新链表
    head2 = tail2;
    while (head2 != nullptr)
    {
        ListNode *temp = head1->next;
        head1->next = head2;
        head1 = temp;
        ListNode *temp2 = head2->next;
        head2->next = head1;
        head2 = temp2;
    }
}