> For the complete documentation index, see [llms.txt](https://hao-fu-1.gitbook.io/oj/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://hao-fu-1.gitbook.io/oj/linked_list/reverse-list/reorder_list.md).

# Reorder List

https\://leetcode.com/problems/reorder-list/description/

## Thoughts

单指针列表顺序变为 *L*0→*Ln*→*L*1→*Ln*-1→*L*2→*Ln*-2→…。把后一半反转，再从head开始隔一个插一个。找中点用快慢指针，再配合dummy和i，j指针方便做merge list。

## Code

```cpp
/*
 * @lc app=leetcode id=143 lang=cpp
 *
 * [143] Reorder List
 */

// @lc code=start
/**
 * 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 == nullptr) return;
        ListNode *dummy = new ListNode(-1), *fast = head->next, *slow = head, *cur = head, *i, *j;
        while (fast != nullptr && fast->next != nullptr) {
            slow = slow->next;
            fast = fast->next;
            fast = fast->next;
        }
        cur = slow->next;
        slow->next = nullptr;
        ListNode *pre = nullptr;
        while (cur != nullptr) {
            auto next = cur->next; 
            cur->next = pre;
            pre = cur;
            cur = next;
        }
        i = head;
        j = pre;
        cur = dummy;
        while (i != nullptr && j != nullptr) {
            cur->next = i;
            i = i->next;
            cur->next->next = j;
            cur = j;
            j = j->next;
        }
        cur->next = i;
    }
};
// @lc code=end


```

## Analysis

TC: O(n), SC： O(1).