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148. Sort List

Difficulty Topics

Description

Given the head of a linked list, return the list after sorting it in ascending order.

 

Example 1:

Input: head = [4,2,1,3]
Output: [1,2,3,4]

Example 2:

Input: head = [-1,5,3,4,0]
Output: [-1,0,3,4,5]

Example 3:

Input: head = []
Output: []

 

Constraints:

  • The number of nodes in the list is in the range [0, 5 * 104].
  • -105 <= Node.val <= 105

 

Follow up: Can you sort the linked list in O(n logn) time and O(1) memory (i.e. constant space)?

Solution

sort-list.py
# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def sortList(self, head: Optional[ListNode]) -> Optional[ListNode]:
        if not head or not head.next: return head

        def merge(head1, head2):
            curr = res = ListNode(-1)

            while head1 and head2:
                if head1.val < head2.val:
                    curr.next = ListNode(head1.val)
                    head1 = head1.next
                else:
                    curr.next = ListNode(head2.val)
                    head2 = head2.next

                curr = curr.next

            curr.next = head1 or head2

            return res.next

        pre, slow, fast = None, head, head

        while fast and fast.next:
            pre, slow, fast = slow, slow.next, fast.next.next

        pre.next = None

        return merge(self.sortList(head), self.sortList(slow))