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}.

算法：

1、找到中间结点，需要考虑奇偶性

2、将后一半链表反序

3、将两个链表按照要求合并

code：

class Solution {
public:
    int lenList(ListNode* head){
        ListNode* p =head;
        int count = 0;
        while(p!=NULL){
            count++;
            p = p->next;
        }
        return count;
    }
    void reorderList(ListNode* head) {

        ListNode* reHead ;
        int length = lenList(head);
        if(length <2)
            return;
        int mid = length/2;
        ListNode *first = head;
        for (int i = 0; i < mid - 1 ; ++i) {
            if(first->next!=NULL)
                first = first->next;
        }

        ListNode *p = first->next;
        first->next = NULL;
        ListNode *q = p->next;
        p->next = NULL;
        ListNode* r;
        while(q!=NULL){
            r = q->next;
            q->next = p;
            p = q;
            q = r;
        }
        reHead = p;
        ListNode *p1 = head;
        ListNode *p2 = reHead;
        while(p1!=NULL){
            q = p1->next;
            r = p2->next;
            p1->next = p2;
            if(q!=NULL)
                p2->next = q;
            p1 = q;
            p2 = r;
        }
    }
};