# Reorder List

### 描述​

Given a singly linked list $L: L_0 \rightarrow L_1 \rightarrow \cdots \rightarrow L_{n-1} \rightarrow L_n$, reorder it to: $L_0 \rightarrow L_n \rightarrow L_1 \rightarrow L_{n-1} \rightarrow L_2 \rightarrow L_{n-2} \rightarrow \cdots$

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}.

### 代码​

// Reorder List// 时间复杂度O(n)，空间复杂度O(1)class Solution {    public void reorderList(ListNode head) {        if (head == null || head.next == null) return;        ListNode slow = head, fast = head, prev = null;        while (fast != null && fast.next != null) {            prev = slow;            slow = slow.next;            fast = fast.next.next;        }        prev.next = null; // cut at middle        slow = reverse(slow);        // merge two lists        ListNode curr = head;        while (curr.next != null) {            ListNode tmp = curr.next;            curr.next = slow;            slow = slow.next;            curr.next.next = tmp;            curr = tmp;        }        curr.next = slow;    }    ListNode reverse(ListNode head) {        if (head == null || head.next == null) return head;        ListNode prev = head;        for (ListNode curr = head.next, next = curr.next; curr != null;            prev = curr, curr = next, next = next != null ? next.next : null) {                curr.next = prev;        }        head.next = null;        return prev;    }}