链表合并排序的实现

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

def merge_sort(head):
    if not head or not head.next:
        return head
    
    # 使用快慢指针找到链表的中点
    slow, fast = head, head.next
    while fast and fast.next:
        slow = slow.next
        fast = fast.next.next
    
    # 分割链表
    mid = slow.next
    slow.next = None
    
    # 递归排序左右两部分
    left = merge_sort(head)
    right = merge_sort(mid)
    
    # 合并两个有序链表
    return merge(left, right)

def merge(left, right):
    dummy = ListNode()
    current = dummy
    
    while left and right:
        if left.val < right.val:
            current.next = left
            left = left.next
        else:
            current.next = right
            right = right.next
        current = current.next
    
    # 将剩余的节点连接到合并链表的末尾
    if left:
        current.next = left
    if right:
        current.next = right
    
    return dummy.next