面试题：有一个链表，奇数位升序偶数位降序，如何将链表变成升序

一、概述

有一个链表，奇数位升序偶数位降序，如何将链表变成升序。

二、思路

1）将链表根据奇偶顺序拆成两个链表

2）将偶数位拆成的链表反转

3）将两个链表合并

三、代码

private static ListNode[] splitList(ListNode head) {
    ListNode cur = head;

    ListNode head1 = null;
    ListNode head2 = null;
    ListNode cur1 = null;
    ListNode cur2 = null;
    int num = 1;

    while (head != null) {
        if (num % 2 == 1) {
            if (cur1 != null) {
                cur1.next = head;
                cur1 = cur1.next;
            } else {
                cur1 = head;
                head1 = cur1;
            }
        } else {
            if (cur2 != null) {
                cur2.next = head;
                cur2 = cur2.next;
            } else {
                cur2 = head;
                head2 = cur2;
            }
        }

        head = head.next;
        num++;
    }

    cur1.next = null;
    cur2.next = null;
    ListNode[] heads = new ListNode[]{head1, head2};
    return heads;
}

private static ListNode reverseList(ListNode head) {
    ListNode pre = null;
    ListNode cur = head;

    while (cur != null) {
        ListNode next = cur.next;
        cur.next = pre;
        pre = cur;
        cur = next;
    }

    return pre;
}

private static ListNode mergeLists(ListNode head1, ListNode head2) {
    if (head1 == null && head2 == null) {
        return null;
    }

    if (head1 == null || head2 == null) {
        return head1 == null ? head2 : head1;
    }

    ListNode first = new ListNode(-1);
    ListNode cur = first;

    while (head1 != null && head2 != null) {
        if (head1.val < head2.val) {
            cur.next = head1;
            head1 = head1.next;
        } else {
            cur.next = head2;
            head2 = head2.next;
        }
    }

    cur.next = head1 == null ? head2 : head1;

    return first.next;
}