链表排序(Sort List)

链表排序所用到的语言和方法：

1、语言：Python
2、时间复杂度O(nlogn)
3、空间复杂度为常数
4、用的排序算法：二路归并排序

具体实现算法如下所示：

 class ListNode(object):
     def __init__(self, x):
         self.val = x
         self.next = None

def sortList(head):
        if head == None or head.next == None:
            return head

        pre = None
        slow = head
        fast = head

        while fast != None and fast.next != None:
            pre = slow
            slow = slow.next
            fast = fast.next.next

        pre.next = None

        left = sortList(head)
        right = sortList(slow)

        return merge(left,right)

    def merge(left,right):
        res = ListNode(0)
        temp = res
        while left != None and right != None:
            if left.val <= right.val:
                temp.next = left
                temp = temp.next
                left = left.next
            else:
                temp.next = right
                temp = temp.next
                right = right.next

        if left != None:
            temp.next = left

        if right != None:
            temp.next = right

        return res.next