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);
}
};