leetcode[143]Reorder List

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}.

/**
 * 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==NULL||head->next==NULL||head->next->next==NULL)return;
    ListNode * slow=head;
    ListNode * fast=head;
    ListNode * pres=NULL;
    while(fast&&fast->next)
    {
        fast=fast->next->next;
        pres=slow;
        slow=slow->next;
    }
    pres->next=NULL;
    ListNode *tmp=new ListNode(0);
    tmp->next=slow;
    ListNode *last=slow;
    slow=slow->next;
    ListNode *p=NULL;
    while(slow&&slow->next)
    {
        p=tmp->next;
        tmp->next=slow;
        ListNode *later=slow->next;
        slow->next=p;
        slow=later;
    }
    if(slow)
    {
        p=tmp->next;
        tmp->next=slow;
        slow->next=p;
    }
    last->next=NULL;
    slow=tmp->next;
    fast=head;
    ListNode *f=NULL;
    while(fast&&slow)
    {
        p=slow->next;
        f=fast->next;
        fast->next=slow;
        if(f==NULL)
        {
            slow->next=p;
            break;
        }
        slow->next=f;
        fast=f;
        slow=p;
    }
}
/**
    void reorderList(ListNode *head) {
        if(head==NULL)return;
        vector<ListNode *> tmp;
        ListNode *p=head;
        while(p)
        {
            tmp.push_back(p);
            p=p->next;
        }
        int left=0,right=tmp.size()-1;
        while(left<right)
        {
            tmp[left]->next=tmp[right];
            left++;
            tmp[right]->next=tmp[left];
            right--;
            tmp[left]->next=tmp[right];
        }
        tmp[left]->next=NULL;
    }
*/
};