Python与数据结构[0] -> 链表/LinkedList[2] -> 链表有环与链表相交判断的 Python 实现

链表有环与链表相交判断的 Python 实现

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目录

1. 有环链表
2. 相交链表

 

1 有环链表

判断链表是否有环可以参考链接，

有环链表主要包括以下几个问题（C语言描述）：

1. 判断环是否存在： 可以使用追赶方法，定义两个指针slow和fast，分别以1步和2步前进，若存在环则两者会相遇，否则fast遇到NULL时则退出；
2. 获取环的长度：若存在环，则以相遇点为起点，fast和slow再次开始前进，第二次碰相遇slow走过的步数（1圈）即为环长度；
3. 找出入环点：相遇点到连接点的距离 = 头指针到连接点的距离，因此，让头指针和slow同时开始前进，相遇的点即为连接点；
4. 带环链表长度：问题3的连接点与头指针长度 + 问题2的环长度即为总长。

下面为关于有环链表几个问题的具体实现代码，

完整代码

 1 from linked_list import LinkedList
 2 
 3 
 4 def check_loop(chain):
 5     has_loop, entry_node, loop_length, chain_length = False, None, 0, 0
 6 
 7     # Get header for fast and slow
 8     step = 0
 9     fast = slow = head = chain.header
10     while fast and fast.next:
11         fast = fast.next.next
12         slow = slow.next
13         step += 1
14         # Note:
15         # Do remember to use ,is' rather than '==' here (assure the id is same).
16         if fast is slow:    
17             break
18     has_loop = not(fast is None or fast.next is None)
19     pass_length = (step * 2) if fast is None else (step * 2 + 1)
20 
21     if has_loop:
22         step = 0
23         while True:
24             if head is slow:
25                 entry_node = slow
26                 pass_length = step
27             if not entry_node:
28                 head = head.next
29             fast = fast.next.next
30             slow = slow.next
31             step += 1
32             if fast is slow:
33                 break
34         loop_length = step
35 
36     chain_length = pass_length + loop_length
37     return has_loop, entry_node, loop_length, chain_length
38 
39 
40 if __name__ == '__main__':
41     print('------------ Loop check ------------------')
42     print('''
43     0 -> 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7 -> 8 -> 9
44     ''')
45     loop_chain = LinkedList(range(10))
46     print('Linked list has loop: %s, entry node: %s, loop length: %s, chain length: %s' % check_loop(loop_chain))
47 
48     # Create a loop for linked list.
49     print('''
50                     _____________________________
51                    |                             |
52                    V                             |
53     0 -> 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7 -> 8 -> 9
54     ''')
55     node_9 = loop_chain.find(9)
56     node_3 = loop_chain.find(3)
57     node_9.next = node_3
58     print('Linked list has loop: %s, entry node: %s, loop length: %s, chain length: %s' % check_loop(loop_chain))

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