题目

给你链表的头结点 head ，请将其按 升序 排列并返回 排序后的链表 。

示例 1：

输入：head = [4,2,1,3]
输出：[1,2,3,4]

代码

第一次使用朴素快速排序，基准值为头节点值，居然超时了，然后又改进成了随机基准的快排，以下是两个版本的代码

# 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]:
        return self.dfs(head)[0]
	
	 #	朴素快速排序
    def dfs(self,head):
        if(not head): return None,None

        smaller_dummy = ListNode(-1)
        smaller_node  = smaller_dummy
        bigger_dummy = ListNode(-1)
        bigger_node  = bigger_dummy
        node = head.next
        while(node):
            if(node.val > head.val):
                bigger_node.next = node
                bigger_node = bigger_node.next
            else:
                smaller_node.next = node
                smaller_node = smaller_node.next
            node = node.next

        smaller_node.next = None
        bigger_node.next = None
        
        smaller_head,smaller_tail = self.dfs(smaller_dummy.next)
        bigger_head,bigger_tail = self.dfs(bigger_dummy.next)


        if(smaller_head):
            smaller_tail.next = head
            head.next = bigger_head
            return smaller_head,bigger_tail or head
        else:
            head.next = bigger_head
            return head,bigger_tail or head

# 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):
        return self.dfs(head)[0]

    # 随机基准数的快排
    def dfs(self,head,low=-100000,high=100000):
        if(not head): return None,None
        
        smaller_dummy = ListNode(-1)
        smaller_node  = smaller_dummy
        bigger_dummy = ListNode(-1)
        bigger_node  = bigger_dummy
        pivot_dummy  = ListNode(-1)
        pivot_node   = pivot_dummy
        node = head
        
        pivot_num = random.randint(low, high)

        while(node):
            if(node.val > pivot_num):
                bigger_node.next = node
                bigger_node = bigger_node.next
            elif(node.val < pivot_num):
                smaller_node.next = node
                smaller_node = smaller_node.next
            else:
                pivot_node.next = node    
                pivot_node = pivot_node.next

            node = node.next

        smaller_node.next = None
        bigger_node.next = None
        
        smaller_head,smaller_tail = self.dfs(smaller_dummy.next,low,pivot_num)
        bigger_head,bigger_tail = self.dfs(bigger_dummy.next,pivot_num,high)
        
        if(pivot_dummy.next):
            if(smaller_head):
                smaller_tail.next = pivot_dummy.next
            else:
                smaller_head = pivot_dummy.next
            smaller_tail = pivot_node

        if(smaller_tail): smaller_tail.next = bigger_head

        return smaller_head or bigger_head,bigger_tail or smaller_tail