Reorder List

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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}.

class Solution {public:    void reorderList(ListNode* head) {        ListNode *second_head=findhalf(head);        ListNode *reverse_head=reverse(second_head);        ListNode *late;        while(reverse_head){            late=reverse_head->next;            reverse_head->next=head->next;            head->next=reverse_head;            head=head->next->next;            reverse_head=late;        }    }    ListNode* findhalf(ListNode * head){        if(!head) return NULL;        ListNode *fast=head,*slow=head;        while(fast&&fast->next){            slow=slow->next;            fast=fast->next->next;        }        ListNode* second_head=slow->next;//注这里不能返回slow        slow->next=NULL;//要加这句，不然原head就还是长长的一条        return second_head;    }    ListNode * reverse(ListNode *head){        if(!head) return NULL;        ListNode*pre=new ListNode(-1);        pre->next=head;        while(head->next){            ListNode *p=head->next;            head->next=p->next;            p->next=pre->next;            pre->next=p;        }        return pre->next;    }};

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