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) return;

        ListNode* p1=head;

        ListNode* p2=head;

        

        stack<ListNode*> stk;

        while(true)

        {

            p1=p1->next;

            p2=p2->next;

            if(p2!=NULL) p2=p2->next;

            if(p2==NULL) break;

        }

        while(p1!=NULL)

        {

            stk.push(p1);

            p1=p1->next;

        }

        p1=head;

        while(stk.empty()==false)

        {

            p2=p1->next;

            p1->next=stk.top();stk.pop();

            p1=p1->next;

            p1->next=p2;

            p1=p2;

        }

        if(p1!=NULL) p1->next=NULL;

    }

};