leetcode 148. 排序链表

leetcode 148. 排序链表

148. 排序链表
     在 O(n log n) 时间复杂度和常数级空间复杂度下，对链表进行排序。

示例 1:

输入: 4->2->1->3
输出: 1->2->3->4
示例 2:

输入: -1->5->3->4->0
输出: -1->0->3->4->5

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def sortList(self, head: ListNode) -> ListNode:
        if not head or not head.next:
            return head
        mid = self.getmid(head)
        rhead = mid.next
        mid.next = None
        return self.mergesort(self.sortList(head),self.sortList(rhead))
    def getmid(self,head):
        if not head or not head.next:
            return None
        slow = fast = head
        while fast.next and fast.next.next:
            fast = fast.next.next
            slow = slow.next
        return slow
    def mergesort(self,lhead,rhead):
        dummy = cur = ListNode(0)
        while lhead and rhead:
            if lhead.val <= rhead.val:
                cur.next = lhead
                lhead = lhead.next
            else:
                cur.next = rhead
                rhead = rhead.next
            cur = cur.next
        cur.next = lhead or rhead
        return dummy.next