描述
Sort a linked list in O(n log n) time using constant space complexity.
分析
要求时间O(nlogn)，上一题的插入排序就不能用了，可以使用归并排序，先split成两个链表，分别递归排序，再归并。可以用到我们之前的Merge Two Sorted List。
代码
归并排序
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class (object):
def mergeTwoLists(self, l1, l2):
dummy = ListNode(-1)
dummy.next = l1
prev = dummy
while l1 and l2:
if l1.val > l2.val:
prev.next = l2
prev = l2
l2 = l2.next
else:
prev.next = l1
prev = l1
l1 = l1.next
if not l1:
prev.next = l2
else:
prev.next = l1
return dummy.next
def splitList(self, head):
h1 = head
h2 = head.next
p1 = h1
p2 = h2
while p2:
p1.next = p2.next
if p2.next:
p2.next = p2.next.next
p1 = p1.next
p2 = p2.next
return h1, h2
def sortList(self, head):
"""
:type head: ListNode
:rtype: ListNode
"""
if not head or not head.next:
return head
l1, l2 = self.splitList(head)
l1 = self.sortList(l1)
l2 = self.sortList(l2)
return self.mergeTwoLists(l1, l2)
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