找到给定链表的中间

给定一个单向链表，找到链表的中间部分。例如，如果给定的链表是 1->2->3->4->5，那么输出应该是 3。
如果有偶数个节点，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。一个指针一个一个地移动，另一个指针两个一个地移动。当快指针到达末尾时，慢指针会到达链表的中间。

下图显示了 printMiddle函数在代码中的工作方式：

Java

// C++ program for the above approach
 
#include 
using namespace std;
 
class Node{
    public:
        int data;
        Node *next;
};
 
class NodeOperation{
  public:
   
    // Function to add a new node
    void pushNode(class Node** head_ref,int data_val){
       
        // Allocate node
        class Node *new_node = new Node();
         
         // Put in the data
        new_node->data = data_val;
         
        // Link the old list off the new node
        new_node->next = *head_ref;
         
        // move the head to point to the new node
        *head_ref = new_node;
    }
     
    // A utility function to print a given linked list
    void printNode(class Node *head){
        while(head != NULL){
            cout <data << "->";
            head = head->next;
        }
        cout << "NULL" << endl;
    }
     
    void printMiddle(class Node *head){
        struct Node *slow_ptr = head;
        struct Node *fast_ptr = head;
  
        if (head!=NULL)
        {
            while (fast_ptr != NULL && fast_ptr->next != NULL)
            {
                fast_ptr = fast_ptr->next->next;
                slow_ptr = slow_ptr->next;
            }
            cout << "The middle element is [" << slow_ptr->data << "]" << endl;
        }
    }
};
 
// Driver Code
int main(){
    class Node *head = NULL;
    class NodeOperation *temp = new NodeOperation();
    for(int i=5; i>0; i--){
        temp->pushNode(&head, i);
        temp->printNode(head);
        temp->printMiddle(head);
    }
    return 0;
}

C

// C program to find middle of linked list
#include
#include
 
/* Link list node */
struct Node
{
    int data;
    struct Node* next;
};
 
/* Function to get the middle of the linked list*/
void printMiddle(struct Node *head)
{
    struct Node *slow_ptr = head;
    struct Node *fast_ptr = head;
 
    if (head!=NULL)
    {
        while (fast_ptr != NULL && fast_ptr->next != NULL)
        {
            fast_ptr = fast_ptr->next->next;
            slow_ptr = slow_ptr->next;
        }
        printf("The middle element is [%d]\n\n", slow_ptr->data);
    }
}
 
void push(struct Node** head_ref, int new_data)
{
    /* allocate node */
    struct Node* new_node =
        (struct Node*) malloc(sizeof(struct Node));
 
    /* put in the data */
    new_node->data = new_data;
 
    /* link the old list off the new node */
    new_node->next = (*head_ref);
 
    /* move the head to point to the new node */
    (*head_ref) = new_node;
}
 
// A utility function to print a given linked list
void printList(struct Node *ptr)
{
    while (ptr != NULL)
    {
        printf("%d->", ptr->data);
        ptr = ptr->next;
    }
    printf("NULL\n");
}
 
/* Driver program to test above function*/
int main()
{
    /* Start with the empty list */
    struct Node* head = NULL;
    int i;
 
    for (i=5; i>0; i--)
    {
        push(&head, i);
        printList(head);
        printMiddle(head);
    }
 
    return 0;
}

Java

// Java program to find middle of linked list
class LinkedList
{
    Node head; // head of linked list
 
    /* Linked list node */
    class Node
    {
        int data;
        Node next;
        Node(int d)
        {
            data = d;
            next = null;
        }
    }
 
    /* Function to print middle of linked list */
    void printMiddle()
    {
        Node slow_ptr = head;
        Node fast_ptr = head;
         
            while (fast_ptr != null && fast_ptr.next != null)
            {
                fast_ptr = fast_ptr.next.next;
                slow_ptr = slow_ptr.next;
            }
            System.out.println("The middle element is [" +
                                slow_ptr.data + "] \n");
         
    }
 
    /* Inserts a new Node at front of the list. */
    public void push(int new_data)
    {
        /* 1 & 2: Allocate the Node &
                  Put in the data*/
        Node new_node = new Node(new_data);
 
        /* 3. Make next of new Node as head */
        new_node.next = head;
 
        /* 4. Move the head to point to new Node */
        head = new_node;
    }
 
    /* This function prints contents of linked list
       starting from  the given node */
    public void printList()
    {
        Node tnode = head;
        while (tnode != null)
        {
            System.out.print(tnode.data+"->");
            tnode = tnode.next;
        }
        System.out.println("NULL");
    }
 
    public static void main(String [] args)
    {
        LinkedList llist = new LinkedList();
        for (int i=5; i>0; --i)
        {
            llist.push(i);
            llist.printList();
            llist.printMiddle();
        }
    }
}
// This code is contributed by Rajat Mishra

C#

// C# program to find middle of linked list
using System;
 
class LinkedList{
     
// Head of linked list 
Node head;
 
// Linked list node
class Node
{
    public int data;
    public Node next;
     
    public Node(int d)
    {
        data = d;
        next = null;
    }
}
 
// Function to print middle of linked list
void printMiddle()
{
    Node slow_ptr = head;
    Node fast_ptr = head;
     
    if (head != null)
    {
        while (fast_ptr != null &&
               fast_ptr.next != null)
        {
            fast_ptr = fast_ptr.next.next;
            slow_ptr = slow_ptr.next;
        }
        Console.WriteLine("The middle element is [" +
                          slow_ptr.data + "] \n");
    }
}
 
// Inserts a new Node at front of the list.
public void push(int new_data)
{
     
    /* 1 & 2: Allocate the Node &
              Put in the data*/
    Node new_node = new Node(new_data);
 
    /* 3. Make next of new Node as head */
    new_node.next = head;
 
    /* 4. Move the head to point to new Node */
    head = new_node;
}
 
// This function prints contents of linked
// list starting from  the given node
public void printList()
{
    Node tnode = head;
    while (tnode != null)
    {
        Console.Write(tnode.data + "->");
        tnode = tnode.next;
    }
    Console.WriteLine("NULL");
}
 
// Driver code
static public void Main()
{
    LinkedList llist = new LinkedList();
    for(int i = 5; i > 0; --i)
    {
        llist.push(i);
        llist.printList();
        llist.printMiddle();
    }
}
}
 
// This code is contributed by Dharanendra L V.

Python3

# Python3 program to find middle of linked list
# Node class
class Node:
   
    # Function to initialise the node object
    def __init__(self, data):
        self.data = data  # Assign data
        self.next = None  # Initialize next as null
   
   
# 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)
 
# Code execution starts here
if __name__=='__main__':
   
    # Start with the empty list
    llist = LinkedList()
   
    for i in range(5, 0, -1):
        llist.push(i)
        llist.printList()
        llist.printMiddle()
 
 # Code is contributed by Kumar Shivam (kshivi99)

Javascript

 
// JavaScript program to find
// middle of linked list
var head; // head of linked list
 
    /* Linked list node */
    class Node {
        constructor(val) {
            this.data = val;
            this.next = null;
        }
    }
 
    /* Function to print middle
    of linked list */
    function printMiddle()
    {
   var slow_ptr = head;
   var fast_ptr = head;
        if (head != null)
        {
            while (fast_ptr != null &&
            fast_ptr.next != null)
            {
                fast_ptr = fast_ptr.next.next;
                slow_ptr = slow_ptr.next;
            }
            document.write(
"The middle element is [" + slow_ptr.data + "] <br/><br/>"
            );
        }
    }
 
    /* Inserts a new Node at front of the list. */
     function push(new_data) {
        /*
         * 1 & 2: Allocate the Node & Put in the data
         */
var new_node = new Node(new_data);
 
        /* 3. Make next of new Node as head */
        new_node.next = head;
 
        /* 4. Move the head to point to new Node */
        head = new_node;
    }
 
    /*
     * This function prints contents of linked
     list starting from the given node
     */
     function printList() {
     var tnode = head;
        while (tnode != null) {
            document.write(tnode.data + "->");
            tnode = tnode.next;
        }
        document.write("NULL<br/>");
    }
 
     
        for (i = 5; i > 0; --i) {
            push(i);
            printList();
            printMiddle();
        }
 
// This code is contributed by todaysgaurav
 

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 开始遍历列表，遍历时递增计数器并在计数器为奇数时将 mid 更改为 mid->next。所以 mid 只会移动列表总长度的一半。
感谢 Narendra Kangralkar 提出这种方法。

C++

#include 
using namespace std;
 
// Link list node
struct node
{
    int data;
    struct node* next;
};
 
// Function to get the middle of
// the linked list
void printMiddle(struct node* head)
{
    int count = 0;
    struct node* mid = head;
 
    while (head != NULL)
    {
         
        // Update mid, when 'count'
        // is odd number
        if (count & 1)
            mid = mid->next;
 
        ++count;
        head = head->next;
    }
 
    // If empty list is provided
    if (mid != NULL)
        printf("The middle element is [%d]\n\n",
                mid->data);
}
 
void push(struct node** head_ref, int new_data)
{
     
    // Allocate node
    struct node* new_node = (struct node*)malloc(
        sizeof(struct node));
 
    // Put in the data
    new_node->data = new_data;
 
    // Link the old list off the new node
    new_node->next = (*head_ref);
 
    // Move the head to point to
    // the new node
    (*head_ref) = new_node;
}
 
// A utility function to print
// a given linked list
void printList(struct node* ptr)
{
    while (ptr != NULL)
    {
        printf("%d->", ptr->data);
        ptr = ptr->next;
    }
    printf("NULL\n");
}
 
// Driver code
int main()
{
     
    // Start with the empty list
    struct node* head = NULL;
    int i;
 
    for(i = 5; i > 0; i--)
    {
        push(&head, i);
        printList(head);
        printMiddle(head);
    }
    return 0;
}
 
// This code is contributed by ac121102

C

#include 
#include 
 
/* Link list node */
struct node {
    int data;
    struct node* next;
};
 
/* Function to get the middle of the linked list*/
void printMiddle(struct node* head)
{
    int count = 0;
    struct node* mid = head;
 
    while (head != NULL) {
        /* update mid, when 'count' is odd number */
        if (count & 1)
            mid = mid->next;
 
        ++count;
        head = head->next;
    }
 
    /* if empty list is provided */
    if (mid != NULL)
        printf("The middle element is [%d]\n\n", mid->data);
}
 
void push(struct node** head_ref, int new_data)
{
    /* allocate node */
    struct node* new_node
        = (struct node*)malloc(sizeof(struct node));
 
    /* put in the data  */
    new_node->data = new_data;
 
    /* link the old list off the new node */
    new_node->next = (*head_ref);
 
    /* move the head to point to the new node */
    (*head_ref) = new_node;
}
 
// A utility function to print a given linked list
void printList(struct node* ptr)
{
    while (ptr != NULL) {
        printf("%d->", ptr->data);
        ptr = ptr->next;
    }
    printf("NULL\n");
}
 
/* Driver program to test above function*/
int main()
{
    /* Start with the empty list */
    struct node* head = NULL;
    int i;
 
    for (i = 5; i > 0; i--) {
        push(&head, i);
        printList(head);
        printMiddle(head);
    }
 
    return 0;
}

Java

class GFG{
 
static Node head;
 
// Link list node
class Node
{
    int data;
    Node next;
 
    // Constructor
    public Node(Node next, int data)
    {
        this.data = data;
        this.next = next;
    }
}
 
// Function to get the middle of
// the linked list
void printMiddle(Node head)
{
    int count = 0;
    Node mid = head;
 
    while (head != null)
    {
 
        // Update mid, when 'count'
        // is odd number
        if ((count % 2) == 1)
            mid = mid.next;
 
        ++count;
        head = head.next;
    }
 
    // If empty list is provided
    if (mid != null)
        System.out.println("The middle element is [" +
                            mid.data + "]\n");
}
 
void push(Node head_ref, int new_data)
{
     
    // Allocate node
    Node new_node = new Node(head_ref, new_data);
 
    // Move the head to point to the new node
    head = new_node;
}
 
// A utility function to print a
// given linked list
void printList(Node head)
{
    while (head != null)
    {
        System.out.print(head.data + "-> ");
        head = head.next;
    }
    System.out.println("null");
}
 
// Driver code
public static void main(String[] args)
{
    GFG ll = new GFG();
 
    for(int i = 5; i > 0; i--)
    {
        ll.push(head, i);
        ll.printList(head);
        ll.printMiddle(head);
    }
}
}
 
// This code is contributed by mark_3

蟒蛇3

# Node class
class Node:
    
    # Function to initialise the node object
    def __init__(self, data):
        self.data = data  # Assign data
        self.next = None  # Initialize next as null
    
# 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)
  
# Code execution starts here
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

C＃

using System;
 
public class GFG
{
 
    static Node head;
 
    // Link list node
    public
 
 class Node {
        public
 
 int data;
        public
 
 Node next;
 
        // Constructor
        public Node(Node next, int data) {
            this.data = data;
            this.next = next;
        }
    }
 
    // Function to get the middle of
    // the linked list
    void printMiddle(Node head) {
        int count = 0;
        Node mid = head;
 
        while (head != null) {
 
            // Update mid, when 'count'
            // is odd number
            if ((count % 2) == 1)
                mid = mid.next;
 
            ++count;
            head = head.next;
        }
 
        // If empty list is provided
        if (mid != null)
            Console.WriteLine("The middle element is [" + mid.data + "]\n");
    }
 
    public void Push(Node head_ref, int new_data) {
 
        // Allocate node
        Node new_node = new Node(head_ref, new_data);
 
        // Move the head to point to the new node
        head = new_node;
    }
 
    // A utility function to print a
    // given linked list
    void printList(Node head) {
        while (head != null) {
            Console.Write(head.data + "-> ");
            head = head.next;
        }
        Console.WriteLine("null");
    }
 
    // Driver code
    public static void Main(String[] args) {
        GFG ll = new GFG();
 
        for (int i = 5; i > 0; i--) {
            ll.Push(head, i);
            ll.printList(head);
            ll.printMiddle(head);
        }
    }
}
 
 
// This code is contributed by Rajput-Ji

Javascript

    var head=null;
 
    // Link list node
     class Node {
            constructor(next,val) {
                this.data = val;
                this.next = next;
            }
        }
 
    // Function to get the middle of
    // the linked list
    function printMiddle(head) {
        var count = 0;
var mid = head;
 
        while (head != null) {
 
            // Update mid, when 'count'
            // is odd number
            if ((count % 2) == 1)
                mid = mid.next;
 
            ++count;
            head = head.next;
        }
 
        // If empty list is provided
        if (mid != null)
            document.write("The middle element is [" + mid.data + "]<br/><br/>");
    }
 
    function push(head_ref , new_data) {
 
        // Allocate node
var new_node = new Node(head_ref, new_data);
 
        // Move the head to povar to the new node
        head = new_node;
        return head;
    }
 
    // A utility function to prvar a
    // given linked list
    function printList(head) {
        while (head != null) {
            document.write(head.data + "-> ");
            head = head.next;
        }
        document.write("null<br/>");
    }
 
    // Driver code
     
        for (i = 5; i > 0; i--) {
        head=    push(head, i);
            printList(head);
            printMiddle(head);
        }
 
// This code contributed by gauravrajput1

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]

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