148. Sort List

    /*
     * 148. Sort List 
     * 11.28 by Mingyang 
     * 因为题目要求的是nlogn的时间复杂度，所以我们这边只能用merge sort来排序
     * 注意，这道题目的时候千万不要按照index来分成两边，必须要两个point一快一慢来进行分类
     */
    public static ListNode sortList(ListNode head) {
        if (head == null || head.next == null)
            return head;
        ListNode slow = head, fast = head, firsthalf = head;
        while (fast.next != null && fast.next.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        ListNode secondhalf = slow.next;
        slow.next = null;
        ListNode leftlist = sortList(firsthalf);
        ListNode rightlist = sortList(secondhalf);
        return mergeTwoLists(leftlist, rightlist);
    }