leetcode 【 Sort List 】 python 实现

题目：

Sort a linked list in O(n log n) time using constant space complexity.

代码：oj 测试通过 Runtime: 372 ms

 # Definition for singly-linked list.
 # class ListNode:
 #     def __init__(self, x):
 #         self.val = x
 #         self.next = None

 class Solution:
     # @param head, a ListNode
     # @return a ListNode
     def sortList(self, head):

         if head is None or head.next is None:
             return head

         slow = head
         fast = head

         while fast.next is not None and fast.next.next is not None:
             slow = slow.next
             fast = fast.next.next

         h1 = head
         h2 = slow.next
         slow.next = None
         l1 = self.sortList(h1)
         l2 = self.sortList(h2)
         head = self.mergeTwoLists(l1,l2)

         return head

     # merger two sorted list
     def mergeTwoLists(self, l1, l2):
         if l1 is None:
             return l2
         if l2 is None:
             return l1

         p = ListNode(0)
         dummyhead = p

         while l1 is not None and l2 is not None:
             if l1.val > l2.val:
                 p.next = l2
                 l2 = l2.next
             else:
                 p.next = l1
                 l1 = l1.next
             p = p.next

         if l1 is None:
             p.next = l2
         else:
             p.next = l1

         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。