leetcode---reorder-list---链表中点、逆转

Given a singly linked list L: L 0→L 1→…→L n-1→L n,
reorder it to: L 0→L n →L 1→L n-1→L 2→L n-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}.

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; */class Solution {public:    ListNode *findMid(ListNode *head)    {        ListNode *fast = head;        ListNode *slow = head;        while(fast->next && fast->next->next)        {            fast = fast->next->next;            slow = slow->next;        }        return slow;    }    ListNode *reverse(ListNode *head)    {        if(!head || !head->next)            return head;        ListNode *nextH = head->next;        ListNode *newH = reverse(nextH);        nextH->next = head;        head->next = NULL;        return newH;    }    ListNode *merge(ListNode *l, ListNode *r)    {        ListNode *p = l;        ListNode *q = r;        while(q)        {            ListNode *pNext = p->next;            ListNode *qNext = q->next;            p->next = q;            q->next = pNext;            p = pNext;            q = qNext;        }        return l;    }    void reorderList(ListNode *head)     {        if(!head)            return;        ListNode *m = findMid(head);        ListNode *r = m->next;        m->next = NULL;        ListNode *right = reverse(r);        head = merge(head, right);    }};