148. Sort List

Sort a linked list in O(n log n) time using constant space complexity.

Example 1:

Input: 4->2->1->3
Output: 1->2->3->4

Example 2:

Input: -1->5->3->4->0
Output: -1->0->3->4->5

O(n2)的方法并不能通过

归并排序

class Solution:
    def sortList(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if head is None or head.next is None:
            return head

        #找到中间节点
        fast,slow = head,head
        while fast.next and fast.next.next:
            fast = fast.next.next
            slow = slow.next

        #断开
        fast = slow
        slow = slow.next
        fast.next = None

        l1 = self.sortList(head)
        l2 = self.sortList(slow)
        return self.merge(l1,l2)

    def merge(self,l1,l2):
        pre = tail = ListNode(0)
        while l1 and l2:
            if l1.val < l2.val:
                tail.next = l1
                tail = l1 
                l1 = l1.next
            else:
                tail.next = l2
                tail = l2
                l2 = l2.next
        tail.next = l1 or l2
        return pre.next