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;
}
}