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;
    }
};