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}.

主要考察对链表的熟悉程度，画个图就比较好理解了

三步：1.将链表从中点断开  2.将第二段链表翻转   3.merge

中间边界条件调试了许久...

/** * 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||!head->next) return;        ListNode *slow=head,*fast=head;        while(fast->next&&fast->next->next){            slow=slow->next;            fast=fast->next->next;        }        ListNode *mid=slow->next;        slow->next=NULL;        ListNode *p=NULL;        while(mid){            ListNode *t=mid;            mid=mid->next;            t->next=p;            p=t;        }        mid=p;        p=head;        while(p&&mid){            ListNode *t1=p->next;            ListNode *t2=mid->next;            p->next=mid;            mid->next=t1;            p=t1;            mid=t2;        }    }};