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148. Sort List

Linked List Two Pointers Divide and Conquer Sorting Merge Sort

Problem - Sort List

Medium

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)?

Solutions

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# 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 head is None or head.next is None:
            return head

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

        left, right = head, slow.next
        slow.next = None
        left, right = self.sortList(left), self.sortList(right)
        dummy = ListNode()
        tail = dummy

        while left and right:
            if left.val < right.val:
                tail.next = left
                left = left.next
            else:
                tail.next = right
                right = right.next
            tail = tail.next
        tail.next = left or right
        return dummy.next

Submission Stats:

  • Runtime: 162 ms (71.26%)
  • Memory: 32.8 MB (69.07%)