148.Sort List
===

###### tags: `Medium`,`Sorting`,`Two Pointers`,`Divide and Conquer`
[148. Sort List](https://leetcode.com/problems/sort-list/)

### 題目描述

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

### 範例

##### 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: []

### 解答
#### Python
Linked List 版 MergeSort

# 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
        
        prev, slow, fast = None, head, head
        
        # split
        while fast and fast.next:
            prev, slow, fast = slow, slow.next, fast.next.next
        prev.next = None
        
        left = self.sortList(head)
        right = self.sortList(slow)
        
        # merge
        def merge(left: Optional[ListNode], right: Optional[ListNode]) -> Optional[ListNode]:
            n = ListNode(0)
            ite = n
            
            while left and right:
                if left.val < right.val:
                    ite.next = left
                    left = left.next
                else:
                    ite.next = right
                    right = right.next
                ite = ite.next
                    
            if left:
                ite.next = left
                
            if right:
                ite.next = right
                
            return n.next
        
        return merge(left, right)

> [name=Kobe Bryant] [time= Nov 23, 2022]

### Reference

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