将节点插入链表的中间

给定一个包含n 个节点的链表。问题是在列表中间插入一个带有数据x的新节点。如果n是偶数，则在第(n/2)个节点之后插入新节点，否则在第(n+1)/2个节点之后插入新节点。
例子：

Input : list: 1->2->4->5
        x = 3
Output : 1->2->3->4->5

Input : list: 5->10->4->32->16
        x = 41
Output : 5->10->4->41->32->16

方法一（使用链表的长度）：
使用一次遍历查找链接的节点数或长度。让它成为len 。计算c = (len/2)，如果len是偶数，否则c = (len+1)/2，如果len是奇数。再次遍历前c 个节点并在第c个节点之后插入新节点。

C++

// C++ implementation to insert node at the middle
// of the linked list
#include 
 
using namespace std;
 
// structure of a node
struct Node {
    int data;
    Node* next;
};
 
// function to create and return a node
Node* getNode(int data)
{
    // allocating space
    Node* newNode = (Node*)malloc(sizeof(Node));
 
    // inserting the required data
    newNode->data = data;
    newNode->next = NULL;
    return newNode;
}
 
// function to insert node at the middle
// of the linked list
void insertAtMid(Node** head_ref, int x)
{
    // if list is empty
    if (*head_ref == NULL)
        *head_ref = getNode(x);
    else {
 
        // get a new node
        Node* newNode = getNode(x);
 
        Node* ptr = *head_ref;
        int len = 0;
 
        // calculate length of the linked list
        //, i.e, the number of nodes
        while (ptr != NULL) {
            len++;
            ptr = ptr->next;
        }
 
        // 'count' the number of nodes after which
        //  the new node is to be inserted
        int count = ((len % 2) == 0) ? (len / 2) :
                                    (len + 1) / 2;
        ptr = *head_ref;
 
        // 'ptr' points to the node after which
        // the new node is to be inserted
        while (count-- > 1)
            ptr = ptr->next;
 
        // insert the 'newNode' and adjust the
        // required links
        newNode->next = ptr->next;
        ptr->next = newNode;
    }
}
 
// function to display the linked list
void display(Node* head)
{
    while (head != NULL) {
        cout << head->data << " ";
        head = head->next;
    }
}
 
// Driver program to test above
int main()
{
    // Creating the list 1->2->4->5
    Node* head = NULL;
    head = getNode(1);
    head->next = getNode(2);
    head->next->next = getNode(4);
    head->next->next->next = getNode(5);
 
    cout << "Linked list before insertion: ";
    display(head);
 
    int x = 3;
    insertAtMid(&head, x);
 
    cout << "\nLinked list after insertion: ";
    display(head);
 
    return 0;
}

Java

// Java implementation to insert node
// at the middle of the linked list
import java.util.*;
import java.lang.*;
import java.io.*;
 
class LinkedList
{
    static Node head; // head of list
 
    /* Node Class */
    static class Node {
        int data;
        Node next;
         
        // Constructor to create a new node
        Node(int d) {
            data = d;
            next = null;
        }
    }
 
    // function to insert node at the
    // middle of the linked list
    static void insertAtMid(int x)
    {
        // if list is empty
        if (head == null)
            head = new Node(x);
        else {
            // get a new node
            Node newNode = new Node(x);
 
            Node ptr = head;
            int len = 0;
 
            // calculate length of the linked list
            //, i.e, the number of nodes
            while (ptr != null) {
                len++;
                ptr = ptr.next;
            }
 
            // 'count' the number of nodes after which
            // the new node is to be inserted
            int count = ((len % 2) == 0) ? (len / 2) :
                                        (len + 1) / 2;
            ptr = head;
 
            // 'ptr' points to the node after which
            // the new node is to be inserted
            while (count-- > 1)
                ptr = ptr.next;
 
            // insert the 'newNode' and adjust
            // the required links
            newNode.next = ptr.next;
            ptr.next = newNode;
        }
    }
 
    // function to display the linked list
    static void display()
    {
        Node temp = head;
        while (temp != null)
        {
            System.out.print(temp.data + " ");
            temp = temp.next;
        }
    }
 
    // Driver program to test above
    public static void main (String[] args)
    {
        // Creating the list 1.2.4.5
        head = null;
        head = new Node(1);
        head.next = new Node(2);
        head.next.next = new Node(4);
        head.next.next.next = new Node(5);
         
        System.out.println("Linked list before "+
                           "insertion: ");
        display();
 
        int x = 3;
        insertAtMid(x);
 
        System.out.println("\nLinked list after"+
                           " insertion: ");
        display();
    }
}
 
// This article is contributed by Chhavi

Python3

# Python3 implementation to insert node
# at the middle of a linked list
 
# Node class
class Node:
 
    # constructor to create a new node
    def __init__(self, data):
        self.data = data
        self.next = None
 
# function to insert node at the
# middle of linked list given the head
def insertAtMid(head, x):
 
    if(head == None): #if the list is empty
        head = Node(x)
    else:
         
        # create a new node for the value
        # to be inserted
        newNode = Node(x)
 
        ptr = head
        length = 0
         
        # calcualte the length of the linked
        # list
        while(ptr != None):
            ptr = ptr.next
            length += 1
 
        # 'count' the number of node after which
        # the new node has to be inserted
        if(length % 2 == 0):
            count = length / 2
        else:
            (length + 1) / 2
 
        ptr = head
 
        # move ptr to the node after which
        # the new node has to inserted
        while(count > 1):
            count -= 1
            ptr = ptr.next
 
        # insert the 'newNode' and adjust
        # links accordingly
        newNode.next = ptr.next
        ptr.next = newNode
 
# function to displat the linked list
def display(head):
    temp = head
    while(temp != None):
        print(str(temp.data), end = " ")
        temp = temp.next
 
# Driver Code
 
# Creating the linked list 1.2.4.5
head = Node(1)
head.next = Node(2)
head.next.next = Node(4)
head.next.next.next = Node(5)
 
print("Linked list before insertion: ", end = "")
display(head)
 
# inserting 3 in the middle of the linked list.
x = 3
insertAtMid(head, x)
 
print("\nLinked list after insertion: " , end = "")
display(head)
 
# This code is contributed by Pranav Devarakonda

C#

// C# implementation to insert node
// at the middle of the linked list
using System;
 
public class LinkedList
{
    static Node head; // head of list
 
    /* Node Class */
    public class Node
    {
        public int data;
        public Node next;
         
        // Constructor to create a new node
        public Node(int d)
        {
            data = d;
            next = null;
        }
    }
 
    // function to insert node at the
    // middle of the linked list
    static void insertAtMid(int x)
    {
        // if list is empty
        if (head == null)
            head = new Node(x);
        else
        {
            // get a new node
            Node newNode = new Node(x);
 
            Node ptr = head;
            int len = 0;
 
            // calculate length of the linked list
            //, i.e, the number of nodes
            while (ptr != null)
            {
                len++;
                ptr = ptr.next;
            }
 
            // 'count' the number of nodes after which
            // the new node is to be inserted
            int count = ((len % 2) == 0) ? (len / 2) :
                                        (len + 1) / 2;
            ptr = head;
 
            // 'ptr' points to the node after which
            // the new node is to be inserted
            while (count-- > 1)
                ptr = ptr.next;
 
            // insert the 'newNode' and adjust
            // the required links
            newNode.next = ptr.next;
            ptr.next = newNode;
        }
    }
 
    // function to display the linked list
    static void display()
    {
        Node temp = head;
        while (temp != null)
        {
            Console.Write(temp.data + " ");
            temp = temp.next;
        }
    }
 
    // Driver code
    public static void Main ()
    {
        // Creating the list 1.2.4.5
        head = null;
        head = new Node(1);
        head.next = new Node(2);
        head.next.next = new Node(4);
        head.next.next.next = new Node(5);
         
        Console.WriteLine("Linked list before "+
                        "insertion: ");
        display();
 
        int x = 3;
        insertAtMid(x);
 
        Console.WriteLine("\nLinked list after"+
                        " insertion: ");
        display();
    }
}
 
/* This code contributed by PrinciRaj1992 */

Javascript

 
// Javascript implementation to insert node
// at the middle of the linked list
 
 
    var head; // head of list
 
    /* Node Class */
     class Node {
 
        // Constructor to create a new node
        constructor(d) {
            this.data = d;
            this.next = null;
        }
    }
 
    // function to insert node at the
    // middle of the linked list
    function insertAtMid(x) {
        // if list is empty
        if (head == null)
            head = new Node(x);
        else {
            // get a new node
            var newNode = new Node(x);
 
            var ptr = head;
            var len = 0;
 
            // calculate length of the linked list
            // , i.e, the number of nodes
            while (ptr != null) {
                len++;
                ptr = ptr.next;
            }
 
            // 'count' the number of nodes after which
            // the new node is to be inserted
            var count = ((len % 2) == 0) ? (len / 2) :
            (len + 1) / 2;
            ptr = head;
 
            // 'ptr' points to the node after which
            // the new node is to be inserted
            while (count-- > 1)
                ptr = ptr.next;
 
            // insert the 'newNode' and adjust
            // the required links
            newNode.next = ptr.next;
            ptr.next = newNode;
        }
    }
 
    // function to display the linked list
    function display() {
        var temp = head;
        while (temp != null) {
            document.write(temp.data + " ");
            temp = temp.next;
        }
    }
 
    // Driver program to test above
     
        // Creating the list 1.2.4.5
         
        head = new Node(1);
        head.next = new Node(2);
        head.next.next = new Node(4);
        head.next.next.next = new Node(5);
 
        document.write("Linked list before "
        + "insertion: ");
        display();
 
        var x = 3;
        insertAtMid(x);
 
        document.write("<br/>Linked list after" +
        " insertion: ");
        display();
 
// This code contributed by Rajput-Ji
 

输出：

Linked list before insertion: 1 2 4 5
Linked list after insertion: 1 2 3 4 5

时间复杂度： O(n)
方法2（使用两个指针）：
基于使用两个指针的 tortoise and hare 算法，一个称为slow ，另一个称为fast 。该算法有助于找到链表的中间节点。在这篇文章的前面和黑色拆分过程中有解释。现在，您可以在通过上述过程获得的中间节点之后插入新节点。这种方法只需要对列表进行一次遍历。

C++

// C++ implementation to insert node at the middle
// of the linked list
#include 
 
using namespace std;
 
// structure of a node
struct Node {
    int data;
    Node* next;
};
 
// function to create and return a node
Node* getNode(int data)
{
    // allocating space
    Node* newNode = (Node*)malloc(sizeof(Node));
 
    // inserting the required data
    newNode->data = data;
    newNode->next = NULL;
    return newNode;
}
 
// function to insert node at the middle
// of the linked list
void insertAtMid(Node** head_ref, int x)
{
    // if list is empty
    if (*head_ref == NULL)
        *head_ref = getNode(x);
 
    else {
        // get a new node
        Node* newNode = getNode(x);
 
        // assign values to the slow and fast
        // pointers
        Node* slow = *head_ref;
        Node* fast = (*head_ref)->next;
 
        while (fast && fast->next) {
 
            // move slow pointer to next node
            slow = slow->next;
 
            // move fast pointer two nodes at a time
            fast = fast->next->next;
        }
 
        // insert the 'newNode' and adjust the
        // required links
        newNode->next = slow->next;
        slow->next = newNode;
    }
}
 
// function to display the linked list
void display(Node* head)
{
    while (head != NULL) {
        cout << head->data << " ";
        head = head->next;
    }
}
 
// Driver program to test above
int main()
{
    // Creating the list 1->2->4->5
    Node* head = NULL;
    head = getNode(1);
    head->next = getNode(2);
    head->next->next = getNode(4);
    head->next->next->next = getNode(5);
 
    cout << "Linked list before insertion: ";
    display(head);
 
    int x = 3;
    insertAtMid(&head, x);
 
    cout << "\nLinked list after insertion: ";
    display(head);
 
    return 0;
}

Java

// Java implementation to insert node
// at the middle of the linked list
import java.util.*;
import java.lang.*;
import java.io.*;
 
class LinkedList
{
    static Node head; // head of list
 
    /* Node Class */
    static class Node {
        int data;
        Node next;
         
        // Constructor to create a new node
        Node(int d) {
            data = d;
            next = null;
        }
    }
 
    // function to insert node at the
    // middle of the linked list
    static void insertAtMid(int x)
    {
        // if list is empty
        if (head == null)
        head = new Node(x);
 
        else {
            // get a new node
            Node newNode = new Node(x);
 
            // assign values to the slow
            // and fast pointers
            Node slow = head;
            Node fast = head.next;
 
            while (fast != null && fast.next
                                  != null)
            {
                // move slow pointer to next node
                slow = slow.next;
 
                // move fast pointer two nodes
                // at a time
                fast = fast.next.next;
            }
 
            // insert the 'newNode' and adjust
            // the required links
            newNode.next = slow.next;
            slow.next = newNode;
        }
    }
 
    // function to display the linked list
    static void display()
    {
        Node temp = head;
        while (temp != null)
        {
            System.out.print(temp.data + " ");
            temp = temp.next;
        }
    }
 
    // Driver program to test above
    public static void main (String[] args)
    {
        // Creating the list 1.2.4.5
        head = null;
        head = new Node(1);
        head.next = new Node(2);
        head.next.next = new Node(4);
        head.next.next.next = new Node(5);
         
        System.out.println("Linked list before"+
                           " insertion: ");
        display();
 
        int x = 3;
        insertAtMid(x);
 
        System.out.println("\nLinked list after"+
                           " insertion: ");
        display();
    }
}
 
// This article is contributed by Chhavi

蟒蛇3

# Python implementation to insert node
# at the middle of the linked list
 
# Node Class
class Node :
    def __init__(self, d):
        self.data = d
        self.next = None
         
class LinkedList:
 
    # function to insert node at the
    # middle of the linked list
    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
         
    def insertAtMid(self, x):
         
        # if list is empty
        if (self.head == None):
            self.head = Node(x)
 
        else:
             
            # get a new node
            newNode = Node(x)
 
            # assign values to the slow
            # and fast pointers
            slow = self.head
            fast = self.head.next
 
            while (fast != None and
                   fast.next != None):
                 
                # move slow pointer to next node
                slow = slow.next
 
                # move fast pointer two nodes
                # at a time
                fast = fast.next.next
 
            # insert the 'newNode' and
            # adjust the required links
            newNode.next = slow.next
            slow.next = newNode
 
    # function to display the linked list
    def display(self):
        temp = self.head
        while (temp != None):
            print(temp.data, end = " "),
            temp = temp.next
 
# Driver Code
 
# Creating the list 1.2.4.5
ll = LinkedList()
ll.push(5)
ll.push(4)
ll.push(2)
ll.push(1)
print("Linked list before insertion: "),
ll.display()
 
x = 3
ll.insertAtMid(x)
 
print("\nLinked list after insertion: "),
ll.display()
 
# This code is contributed by prerna saini

C＃

// C# implementation to insert node
// at the middle of the linked list
using System;
 
public class LinkedList
{
    static Node head; // head of list
 
    /* Node Class */
    class Node
    {
        public int data;
        public Node next;
         
        // Constructor to create a new node
        public Node(int d)
        {
            data = d;
            next = null;
        }
    }
 
    // function to insert node at the
    // middle of the linked list
    static void insertAtMid(int x)
    {
        // if list is empty
        if (head == null)
        head = new Node(x);
 
        else
        {
            // get a new node
            Node newNode = new Node(x);
 
            // assign values to the slow
            // and fast pointers
            Node slow = head;
            Node fast = head.next;
 
            while (fast != null && fast.next
                                != null)
            {
                // move slow pointer to next node
                slow = slow.next;
 
                // move fast pointer two nodes
                // at a time
                fast = fast.next.next;
            }
 
            // insert the 'newNode' and adjust
            // the required links
            newNode.next = slow.next;
            slow.next = newNode;
        }
    }
 
    // function to display the linked list
    static void display()
    {
        Node temp = head;
        while (temp != null)
        {
            Console.Write(temp.data + " ");
            temp = temp.next;
        }
    }
 
    // Driver code
    public static void Main (String[] args)
    {
        // Creating the list 1.2.4.5
        head = null;
        head = new Node(1);
        head.next = new Node(2);
        head.next.next = new Node(4);
        head.next.next.next = new Node(5);
         
        Console.WriteLine("Linked list before"+
                        " insertion: ");
        display();
 
        int x = 3;
        insertAtMid(x);
 
        Console.WriteLine("\nLinked list after"+
                        " insertion: ");
        display();
    }
}
 
// This code is contributed by Rajput-Ji

Javascript

 
// Javascript implementation to insert node
// at the middle of the linked list
 
var head; // head of list
 
    /* Node Class */
     class Node {
 
// Constructor to create a new node
constructor(val) {
    this.data = val;
    this.next = null;
}
}
 
    // function to insert node at the
    // middle of the linked list
    function insertAtMid(x) {
        // if list is empty
        if (head == null)
            head = new Node(x);
 
        else {
            // get a new node
    var newNode = new Node(x);
 
            // assign values to the slow
            // and fast pointers
    var slow = head;
    var fast = head.next;
 
            while (fast != null && fast.next != null)
            {
                // move slow pointer to next node
                slow = slow.next;
 
                // move fast pointer two nodes
                // at a time
                fast = fast.next.next;
            }
 
            // insert the 'newNode' and adjust
            // the required links
            newNode.next = slow.next;
            slow.next = newNode;
        }
    }
 
    // function to display the linked list
      function display() {
       var temp = head;
        while (temp != null) {
            document.write(temp.data + " ");
            temp = temp.next;
        }
    }
 
    // Driver program to test above
     
        // Creating the list 1.2.4.5
        head = null;
        head = new Node(1);
        head.next = new Node(2);
        head.next.next = new Node(4);
        head.next.next.next = new Node(5);
 
        document.write(
        "Linked list before" + " insertion: "
        );
        display();
 
        var x = 3;
        insertAtMid(x);
 
        document.write(
        "<br/>Linked list after" + " insertion: "
        );
        display();
 
// This code is contributed by todaysgaurav
 

输出：

Linked list before insertion: 1 2 4 5
Linked list after insertion: 1 2 3 4 5

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