147. Insertion Sort List

    /*
     * 147. Insertion Sort List 
     * 2016-6-1 by Mingyang 
     * insertion sort的基本思路要有，比较巧的一点是，每处理一个点的时候，把下面一个点用next存起来
     * 这样的话，下次就可以直接对next进行处理，然后里面第二个while循环只用包含一种情况，另一种情况break
     * 实际上就不用写下来了
     */
    public ListNode insertionSortList(ListNode head) {
        if (head == null) {
            return head;
        }
        ListNode helper = new ListNode(0); // new starter of the sorted list
        ListNode cur = head; // the node will be inserted
        ListNode pre = helper; // insert node between pre and pre.next
        ListNode next = null; // the next node will be inserted
        // not the end of input list
        while (cur != null) {
            next = cur.next;
            // find the right place to insert
            while (pre.next != null && pre.next.val < cur.val) {
                pre = pre.next;
            }
            // insert between pre and pre.next
            cur.next = pre.next;
            pre.next = cur;
            // 让pre与cur回归原来的位置
            pre = helper;
            cur = next;
        }
        return helper.next;
    }