leetcode 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}

.

利用快慢指针把链表一分为二，再把后半部分链表求倒序，然后合并前后半部分链表。（一定注意边界条件，没有元素，只有一个元素等情况！）

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
void reorderList(ListNode *head) {
if(head==NULL)return;
ListNode *cur=head,*mid=head,*midnext,*midnextnext;
while(cur){
cur=cur->next;
if(cur)cur=cur->next;
else break;
if(cur)mid=mid->next;
}
midnext=mid->next;
while(midnext&&midnext->next){
midnextnext=midnext->next;
midnext->next=midnextnext->next;
midnextnext->next=mid->next;
mid->next=midnextnext;
}
cur=head;
while(cur!=mid){
midnext=mid->next;
mid->next=midnext->next;
midnext->next=cur->next;
cur->next=midnext;
cur=midnext->next;
}
}
};