链表的归并排序-LeetCode（Python版）

双指针+归并排序！图解排序链表！-知乎

class ListNode(object):
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next
        
class Solution(object):
    def find_mid(self, head):  # 快慢指针
        slow, fast = head, head
        while fast.next and fast.next.next:
            slow = slow.next
            fast = fast.next.next
        mid = slow.next
        slow.next = None
        return head, mid

    def merge(self, left, right):
        new = ListNode()
        node = new
        while left and right:
            if left.val < right.val:
                node.next = left
                node = left
                left = left.next
            else:
                node.next = right
                node = right
                right = right.next
        k = left if left else right
        node.next = k
        return new.next

    def sortList(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if head is None or head.next is None:
            return head
        head, mid = self.find_mid(head)
        head = self.sortList(head)
        mid = self.sortList(mid)
        return self.merge(head, mid)
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