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。