Leetcode#143 Reorder List

原题地址

先把链表分割成前后两半，然后交叉融合

实践证明，凡是链表相关的题目，都应该当成工程类题目做，局部变量、功能函数什么的随便整，代码长了没关系，关键是清楚，不容易出错。

代码：

 ListNode *reverseList(ListNode *head) {
   if (!head) return head;

   ListNode *prev = head;
   head = head->next;
   prev->next = NULL;
   while (head) {
     ListNode *next = head->next;
     head->next = prev;
     prev = head;
     head = next;
   }

   return prev;
 }

 ListNode *mergeList(ListNode *head1, ListNode *head2) {
   ListNode *next1, *next2;
   ListNode *head = head1;

   while (head1 && head2) {
     next1 = head1->next;
     next2 = head2->next;
     head1->next = head2;
     if (next1)
       head2->next = next1;
     head1 = next1;
     head2 = next2;
   }

   return head;
 }

 void reorderList(ListNode *head) {
   if (!head)
     return;

   ListNode *fast = head;
   ListNode *slow = head;
   while (fast && fast->next) {
     fast = fast->next->next;
     slow = slow->next;
   }

   ListNode *head1 = head;
   ListNode *head2 = reverseList(slow->next);
   slow->next = NULL;
   mergeList(head1, head2);
 }