Reorder List

 Givena singly linked list L: L₀→L₁→…→L_n-1→L_n,
reorder it to: L₀→L_n→L₁→L_n-1→L₂→L_n-2→…

You must do this in-place withoutaltering the nodes' values.

For example,
Given {1,2,3,4},reorder it to {1,4,2,3}.

思路：小技巧就是把链表拆车两段，后面的reverse，然后两个链表重新合并。里面考的基础知识比较多，比如如何拆两端，如何reverse一个list，如何合并两个链表。

class Solution {public:  void reorderList(ListNode *head) {        if(head == NULL || head->next == NULL) {            return;        }                ListNode *slow = head;        ListNode *fast = head;                while (fast->next && fast->next->next) {            slow = slow->next;            fast = fast->next->next;        }                ListNode *secondHead = slow->next;        slow->next = NULL; //这个地方写成==check了一个多小时！！！                secondHead = reverseList(secondHead);        ListNode *firstHead = head;                while (secondHead) {            ListNode *tmp1 = firstHead->next;            ListNode *tmp2 = secondHead->next;            firstHead->next = secondHead;            secondHead->next = tmp1;            firstHead = tmp1;            secondHead = tmp2;        }    }        ListNode* reverseList(ListNode *head) {        if (head == NULL || head->next == NULL) {            return head;        }        ListNode *pre = head;        ListNode *curr = head->next;                while (curr) {            ListNode *tmp = curr->next;            curr->next = pre;            pre = curr;            curr = tmp;        }                head->next = NULL;        return pre;            }};