Python程序检测链表中的循环

当需要检测链表中的循环时，定义一种将元素添加到链表中的方法和一种获取链表中元素的方法。另外定义一种方法检查头和尾的值是否相同。根据这个结果，检测循环。

下面是演示代码 −

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示例

class Node:
   def __init__(self, data):
      self.data = data
      self.next = None

class LinkedList_structure:
   def __init__(self):
      self.head = None
      self.last_node = None

   def add_vals(self, data):
      if self.last_node is None:
         self.head = Node(data)
         self.last_node = self.head
      else:
         self.last_node.next = Node(data)
         self.last_node = self.last_node.next

def get_node_val(self, index):
   curr = self.head
   for i in range(index):
      curr = curr.next
      if curr is None:
         return None
      return curr

def check_cycle(my_list):
   slow_val = my_list.head
   fast_val = my_list.head
   while (fast_val != None and fast_val.next != None):
      slow_val = slow_val.next
      fast_val = fast_val.next.next
      if slow_val == fast_val:
         return True
      return False

my_linked_list = LinkedList_structure()
my_list = input('Enter the elements in the linked list ').split()
   for elem in my_list:
my_linked_list.add_vals(int(elem))
my_len = len(my_list)
if my_len != 0:
   vals = '0-' + str(my_len - 1)
   last_ptr = input('Enter the index [' + vals + '] of the node' ' at which the last node has to point'' (Enter nothing to point to None): ').strip()
   if last_ptr == '':
      last_ptr = None
   else:
      last_ptr = my_linked_list.get_node_val(int(last_ptr))
      my_linked_list.last_node.next = last_ptr

if check_cycle(my_linked_list):
   print("The linked list has a cycle")
else:
   print("The linked list doesn't have a cycle")

输出

Enter the elements in the linked list 56 78 90 12 4
Enter the index [0-4] of the node at which the last node has to point (Enter nothing to point to
None):
The linked list doesn't have a cycle