LeetCode 143. 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}.

重排链表，设链表长为n+1，要求第n-m个结点（n,m = 0,1,2,……n）插入到第m个结点后。

其实就是沿着链表中点对称插入重排，而链表的特殊之处就在于不能走回头路，但我们可以把链表后半部分进行逆序，然后把前后两部分的链表合并就ok了。过程：

1-2-3-4-5-6-7-8↓1-2-3-45-6-7-8↓1-2-3-48-7-6-5↓1-8-2-7-3-6-4-5

class Solution {public:    void reorderList(ListNode* head) {        if (!head || !head->next) return;        //找中点        ListNode *slow = head, *fast = head;        while (fast->next && fast->next->next)        {            slow = slow->next;            fast = fast->next->next;        }        //分割链表成两部分        ListNode *midNode = slow->next;        slow->next = NULL;        //翻转链表后半部分        ListNode *pre = midNode, *cur;        while (pre && (cur = pre->next))        {            pre->next = cur->next;            cur->next = midNode;            midNode = cur;        }        //插入        cur = head;        ListNode *subCur;        while ((subCur = midNode))        {            midNode = subCur->next;            subCur->next = cur->next;            cur->next = subCur;            cur = cur->next->next;        }        //return head;    }};