Sort a linked list in O(n log n) time using constant space complexity.
（实现实现复杂度为O(nlog(n))的链表排序）
Example:
1. 归并排序
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class (object):
def merge(self, left, right):
new_head = ListNode(0)
cur, left_i, right_i = new_head, left, right
while left_i and right_i:
if left_i.val < right_i.val:
cur.next = left_i
left_i = left_i.next
else:
cur.next = right_i
right_i = right_i.next
cur = cur.next
if left_i:
cur.next = left_i
else:
cur.next = right_i
return new_head.next
def sortList(self, head):
"""
:type head: ListNode
:rtype: ListNode
"""
if not head or not head.next:
return head
slow, fast = head, head.next
while fast and fast.next:
slow = slow.next
fast = fast.next.next
middle = slow.next
slow.next = None
left = self.sortList(head)
right = self.sortList(middle)
return self.merge(left, right)
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