【Leetcode】【Medium】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}

.

解题思路：

设置一个指针mid指向链表的中间结点；

将mid之后的半个链表进行反转，反转后的链表insert;

将insert中的结点挨个插入前半段链表中；

代码：

/**
* 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 || !head->next->next)
return;
ListNode* ctrl = head;
ListNode* mid = head;
ListNode* insert = NULL;

while (ctrl->next && ctrl->next->next) {
ctrl = ctrl->next->next;
mid = mid->next;
}

insert = reverseList(mid->next);
mid->next = NULL;
ctrl = head;

while (insert) {
ListNode* t = ctrl->next;
ctrl->next = insert;
insert = insert->next;
ctrl->next->next = t;
ctrl = t;
}

return;
}

ListNode* reverseList(ListNode* head) {
if (!head->next)
return head;
ListNode* cur = NULL;
ListNode* next = head;
ListNode* left = NULL;
while (next) {
left = next->next;
next->next = cur;
cur = next;
next = left;
}
return cur;
}
};