LeetCode: Reorder List

Title:

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}.

说明： 先分解链表，分解为前半部分，和后半部分，再反转后半部分链表。最后合并两个链表。

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

void reorderList(ListNode *head) 
{
    ListNode *ptr;
    ListNode *fast,*slow;
    if(head==NULL||head->next==NULL||head->next->next==NULL)
        return ;
    fast=head;
    slow=head;
    while(fast!=NULL&&fast->next!=NULL)
    {
        fast=fast->next->next;
        slow=slow->next;
    }
    ListNode *preverse=slow;
    if(slow==NULL)
        return ;
    ListNode *rehead=preverse->next;
    preverse->next=NULL;
    if(rehead==NULL)
        return ;
    ListNode *ptr1,*ptr2,*ptr3;
    ptr1=rehead;
    ptr2=rehead->next;
    ptr3=rehead->next;
    rehead->next=NULL;
    while(ptr2!=NULL)
    {
        ptr3=ptr3->next;
        ptr2->next=ptr1;
        ptr1=ptr2;
        ptr2=ptr3;
    }
    rehead=ptr1;
    ptr=head;
    while(ptr!=NULL&&rehead!=NULL)
    {
        ptr1=ptr->next;
        ptr2=rehead->next;
        ptr->next=rehead;
		rehead->next=ptr1;
        ptr=ptr1;
        rehead=ptr2;
    }
}