[leetcode] 143. Reorder List 解题报告

题目链接：https://leetcode.com/problems/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}

.

思路：先使用快慢指针将链表从中间分割成两段，然后后半段就地逆置．之后合并插入到前半段链表即可，时间复杂度O(n)。
代码如下：

/**
* 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, *p=head, *q=head;
while(fast->next && fast->next->next)
slow = slow->next, fast = fast->next->next;
fast = slow->next, slow->next = NULL;
p = fast, q = fast->next, fast->next = NULL;
while(q)
{
auto tem = q->next;
q->next = p;
p = q, q = tem;
}
q = head;
while(q && p)
{
auto tem1 = q->next, tem2 = p->next;
p->next = q->next;
q->next = p;
q = tem1, p = tem2;
}
}
};