给定一个单链表L：L0→L1→…→Ln-1→Ln,
重新排列后为： L0→Ln→L1→Ln-1→L2→Ln-2→…
必须在不改变节点的值的情况下进行原地操作。
例如，
给定链表 {1,2,3,4}，按要求重排后为 {1,4,2,3}。
详见：https://leetcode.com/problems/reorder-list/description/
/**
* 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) {
if(head==nullptr||head->next==nullptr)
{
return;
}
ListNode* head1=head;
ListNode* head2=head;
while(head2&&head2->next)
{
head1=head1->next;
head2=head2->next->next;
}
head2=head1->next;
head1->next=nullptr;
ListNode* pre=nullptr;
ListNode* next=nullptr;
while(head2)
{
next=head2->next;
head2->next=pre;
pre=head2;
head2=next;
}
head2=pre;
head1=head;
ListNode* post1,*post2;
while(head1&&head2)
{
post1=head1->next;
post2=head2->next;
head1->next=head2;
head2->next=post1;
head1=post1;
head2=post2;
}
}
};
参考：https://www.cnblogs.com/grandyang/p/4254860.html
