C# Program to add a node before the given node in a Linked List

A LinkedList in C# is a doubly-linked list that allows efficient insertion and deletion of nodes at any position. The AddBefore() method inserts a new node immediately before a specified existing node in the linked list.

Syntax

Following is the syntax for the AddBefore() method −

public LinkedListNode<T> AddBefore(LinkedListNode<T> node, T value)
public LinkedListNode<T> AddBefore(LinkedListNode<T> node, LinkedListNode<T> newNode)

Parameters

• node − The existing LinkedListNode<T> before which to insert the new node.

• value − The value to add to the LinkedList.

• newNode − The new LinkedListNode<T> to add before the specified node.

Return Value

Returns the new LinkedListNode<T> containing the value.

Using AddBefore() with Node Reference

Here's how to add a node before a specific existing node in a LinkedList −

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        string[] students = {"Tim", "Jack", "Henry", "David", "Tom"};
        LinkedList<string> list = new LinkedList<string>(students);
        
        Console.WriteLine("Original LinkedList:");
        foreach (var stu in list) {
            Console.WriteLine(stu);
        }
        
        // Add a node at the end and get its reference
        var newNode = list.AddLast("Kevin");
        
        // Add a new node before the node added above
        list.AddBefore(newNode, "Matt");
        
        Console.WriteLine("\nLinkedList after adding Matt before Kevin:");
        foreach (var stu in list) {
            Console.WriteLine(stu);
        }
    }
}

The output of the above code is −

Original LinkedList:
Tim
Jack
Henry
David
Tom
LinkedList after adding Matt before Kevin:
Tim
Jack
Henry
David
Tom
Matt
Kevin

Using AddBefore() with Find() Method

You can also find an existing node and insert before it −

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        LinkedList<string> cities = new LinkedList<string>();
        cities.AddLast("New York");
        cities.AddLast("London");
        cities.AddLast("Tokyo");
        
        Console.WriteLine("Original list:");
        foreach (var city in cities) {
            Console.WriteLine(city);
        }
        
        // Find the node containing "London"
        var londonNode = cities.Find("London");
        if (londonNode != null) {
            cities.AddBefore(londonNode, "Paris");
        }
        
        Console.WriteLine("\nAfter adding Paris before London:");
        foreach (var city in cities) {
            Console.WriteLine(city);
        }
    }
}

The output of the above code is −

Original list:
New York
London
Tokyo

After adding Paris before London:
New York
Paris
London
Tokyo

Conclusion

The AddBefore() method in C# LinkedList provides an efficient way to insert nodes before specific existing nodes. It requires a reference to the target node and returns the newly created node, maintaining the doubly-linked structure of the list.