[LeetCode]Reorder List

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

三步走，找中间节点，翻转后半部分，前后两部分依次插入

/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
public class Solution {
public void reorderList(ListNode head) {
if (head == null || head.next == null) return;
ListNode mid = head, tail = head.next.next;
while (tail != null && tail.next != null) {
mid = mid.next;
tail = tail.next.next;
}
ListNode rHead = reverse(mid.next);
ListNode head1 = head;
mid.next = null;
ListNode res = new ListNode(0);
while (head1 != null && rHead != null) {
res.next = head1;
// 涉及指针部分，一步一移，如果把下面两行顺序调换就会出错
head1 = head1.next;
res.next.next = rHead;
rHead = rHead.next;
res = res.next.next;
}
}
private ListNode reverse(ListNode node) {
if (node == null) return node;
ListNode cur = node.next, next = null;
node.next = null;
while (cur != null) {
next = cur.next;
cur.next = node;
node = cur;
cur = next;
}
return node;
}
}