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sort list.py
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sort list.py
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'''
Sort a linked list in O(n log n) time using constant space complexity.
'''
'''
Method: divide and conquer O(nlogn) time O(n) space
'''
class Solution(object):
def sortList(self, head):
"""
:type head: ListNode
:rtype: ListNode
"""
if not head or not head.next:
return head
slow = head
fast = head.next
while fast and fast.next: # find middle node
slow = slow.next
fast = fast.next.next
latter = slow.next
slow.next = None
left = self.sortList(head)
right = self.sortList(latter)
return self.comb(left, right)
def comb(self, left, right): # sort the sublist
dummy = ListNode(min(left.val, right.val) - 1)
current = dummy
while left or right:
if not left:
current.next = right
break
if not right:
current.next = left
break
if left.val < right.val:
current.next = left
left = left.next
else:
current.next = right
right = right.next
current = current.next
return dummy.next