原题链接: https://oj.leetcode.com/problems/sort-list/

public ListNode sortList(ListNode head) {
    if (head == null) return null;
    if (head.next == null) return head;

    ListNode slowCursor = head;
    ListNode quickCursor = head;

    while (quickCursor.next != null && quickCursor.next.next != null) {
        slowCursor = slowCursor.next;
        quickCursor = quickCursor.next.next;
    }

    ListNode headRight = slowCursor.next;
    ListNode headLeft = head;
    slowCursor.next = null;

    return mergeTwoLists(sortList(headLeft), sortList(headRight));
}

private ListNode mergeTwoLists(ListNode l1, ListNode l2) {
    ListNode helper = new ListNode(0);
    helper.next = l1;
    ListNode pre = helper;

    while (l1 != null && l2 != null) {
        if (l1.val < l2.val) {
            l1 = l1.next;
        } else {
            ListNode next = l2.next;
            l2.next = pre.next;
            pre.next = l2;
            l2 = next;
        }

        pre = pre.next;
    }

    if (l2 != null) {
        pre.next = l2;
    }

    return helper.next;
}