题目：
python做什么的、Sort a linked list in O(n log n) time using constant space complexity.
python type？代码：oj 测试通过 Runtime: 372 ms
1 # Definition for singly-linked list. 2 # class ListNode: 3 # def __init__(self, x): 4 # self.val = x 5 # self.next = None 6 7 class Solution: 8 # @param head, a ListNode 9 # @return a ListNode 10 def sortList(self, head): 11 12 if head is None or head.next is None: 13 return head 14 15 slow = head 16 fast = head 17 18 while fast.next is not None and fast.next.next is not None: 19 slow = slow.next 20 fast = fast.next.next 21 22 h1 = head 23 h2 = slow.next 24 slow.next = None 25 l1 = self.sortList(h1) 26 l2 = self.sortList(h2) 27 head = self.mergeTwoLists(l1,l2) 28 29 return head 30 31 # merger two sorted list 32 def mergeTwoLists(self, l1, l2): 33 if l1 is None: 34 return l2 35 if l2 is None: 36 return l1 37 38 p = ListNode(0) 39 dummyhead = p 40 41 while l1 is not None and l2 is not None: 42 if l1.val > l2.val: 43 p.next = l2 44 l2 = l2.next 45 else: 46 p.next = l1 47 l1 = l1.next 48 p = p.next 49 50 if l1 is None: 51 p.next = l2 52 else: 53 p.next = l1 54 55 return dummyhead.next
思路：
归并排序的原理无需多说，感觉像小学老师给卷子排序：每个小组的组内成绩由低到高排序；小组长再交给大组长；大组长再交给老师。
小白实现的步骤是先测试通过mergeTwoLists函数，实现两个有序list的合并。详情见http://www.cnblogs.com/xbf9xbf/p/4186905.html这篇日志。
在实现两个有序链表的合并基础上，再在主程序写递归程序。
