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struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};
 
class Solution {
public:
    void reorderList(ListNode* head)
    {
        if(head == nullptr) return;
        int size = 0;
        ListNode *ptr = head;
        while(ptr != nullptr)
        {
            size++;
            ptr = ptr->next;
        }
        if(size <= 2) return;
        int breakpoint = (size + 1) / 2;
        int i = 0;
        ptr = head;
        while( i++ < breakpoint)
        {
            ptr = ptr->next;
        }
        ListNode *ptr2 = head;
        while(ptr2!= nullptr && ptr2->next != ptr)
            ptr2 = ptr2->next;
        if(ptr2 != nullptr)
            ptr2->next = nullptr;
         
        ListNode* newheadof2ndPart = reverseLinkedList(ptr);
        ptr = head;
        for(i = 0; i< breakpoint && ptr != nullptr && newheadof2ndPart != nullptr; i++)
        {
            ListNode*ptmp = ptr->next;
            ListNode*pnextnewhead = newheadof2ndPart->next;
            ptr -> next = newheadof2ndPart;
            newheadof2ndPart->next = ptmp;
            ptr = ptmp;
            newheadof2ndPart = pnextnewhead;
        }
    }
     
    ListNode* reverseLinkedList(ListNode* head)
    {
        if(head == nullptr) return nullptr;
        ListNode* newhead = reverseLinkedList(head->next);
        if(head-> next != nullptr)
        {
            head->next->next = head; //Error, 一定要搞清楚到底是哪个next哦
        }
        if(newhead == nullptr) newhead = head;
        head->next = nullptr;
         
        return newhead;
    }
     
};