用于查找给定链表的中间元素的Python程序

给定一个单链表，找到链表的中间。例如，如果给定的链表是 1->2->3->4->5，那么输出应该是 3。
如果有偶数节点，那么就会有两个中间节点，我们需要打印第二个中间元素。例如，如果给定的链表是 1->2->3->4->5->6，那么输出应该是 4。

方法一：
遍历整个链表并计算编号。的节点。现在再次遍历列表直到 count/2 并返回 count/2 处的节点。

方法二：
使用两个指针遍历链表。将一个指针移动一格，将其他指针移动二格。当快指针到达末尾时，慢指针将到达链表的中间。

下图显示了 printMiddle函数在代码中的工作方式：

Python3

# Python3 program to find middle of 
# the linked list
# Node class 
class Node: 
    
    # Function to initialise the 
    # node object 
    def __init__(self, data): 
     
        # Assign data 
        self.data = data 
  
        # Initialize next as null  
        self.next = None  
    
# Linked List class contains a 
# Node object 
class LinkedList: 
    
    # Function to initialize head 
    def __init__(self): 
        self.head = None
  
    # Function to insert a new node at 
    # the beginning  
    def push(self, new_data):  
        new_node = Node(new_data)  
        new_node.next = self.head  
        self.head = new_node
  
    # Print the linked list
    def printList(self):
        node = self.head
        while node:
            print(str(node.data) + 
                  "->", end = "")
            node = node.next
        print("NULL")
  
    # Function that returns middle.
    def printMiddle(self):
  
        # Initialize two pointers, one will go 
        # one step a time (slow), another two 
        # at a time (fast)
        slow = self.head
        fast = self.head
  
        # Iterate till fast's next is null (fast 
        # reaches end)
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next
          
        # Return the slow's data, which would be 
        # the middle element.
        print("The middle element is ", slow.data)
  
# Driver code
if __name__=='__main__': 
    
    # Start with the empty list 
    llist = LinkedList() 
    
    for i in range(5, 0, -1):
        llist.push(i)
        llist.printList()
        llist.printMiddle()
  
# This code is contributed by Kumar Shivam (kshivi99)

输出：

5->NULL
The middle element is [5]

4->5->NULL
The middle element is [5]

3->4->5->NULL
The middle element is [4]

2->3->4->5->NULL
The middle element is [4]

1->2->3->4->5->NULL
The middle element is [3]

方法三：
将 mid 元素初始化为 head 并将计数器初始化为 0。从 head 遍历列表，同时遍历递增计数器并在计数器为奇数时将 mid 更改为 mid->next。所以中间只会移动列表总长度的一半。
感谢 Narendra Kangralkar 提出这种方法。

Python3

# Python program to implement
# the above approach
  
# Node class 
class Node: 
     
    # Function to initialise the 
    # node object 
    def __init__(self, data):
  
        # Assign data  
        self.data = data  
  
        # Initialize next as null 
        self.next = None  
     
# Linked List class contains a 
# Node object 
class LinkedList: 
     
    # Function to initialize head 
    def __init__(self): 
        self.head = None
   
    # Function to insert a new node at 
    # the beginning  
    def push(self, new_data):  
        new_node = Node(new_data)  
        new_node.next = self.head  
        self.head = new_node
   
    # Print the linked list
    def printList(self):
        node = self.head
        while node:
            print(str(node.data) + 
                      "->", end = "")
            node = node.next
        print("NULL")
   
    # Function to get the middle of
    #  the linked list
    def printMiddle(self):
        count = 0
        mid = self.head
        heads = self.head
    
        while(heads != None):
        
        # Update mid, when 'count'
        # is odd number 
            if count & 1:
                mid = mid.next
            count += 1
            heads = heads.next
              
        # If empty list is provided 
        if mid != None:
            print("The middle element is ", 
                   mid.data)
   
# Driver code
if __name__=='__main__': 
     
    # Start with the empty list 
    llist = LinkedList() 
     
    for i in range(5, 0, -1):
        llist.push(i)
        llist.printList()
        llist.printMiddle()
   
 # This code is contributed by Manisha_Ediga

输出：

5->NULL
The middle element is [5]

4->5->NULL
The middle element is [5]

3->4->5->NULL
The middle element is [4]

2->3->4->5->NULL
The middle element is [4]

1->2->3->4->5->NULL
The middle element is [3]

有关详细信息，请参阅有关查找给定链接列表的中间的完整文章！