DSA using Java - Linked List

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Linked List Basics

Linked List is a sequence of links which contains items. Each link contains a connection to another link. Linked list the second most used data structure after array. Following are important terms to understand the concepts of Linked List.

  • Link − Each Link of a linked list can store a data called an element.

  • Next − Each Link of a linked list contain a link to next link called Next.

  • LinkedList − A LinkedList contains the connection link to the first Link called First.

Linked List Representation

Linked List

As per above shown illustration, following are the important points to be considered.

  • LinkedList contains an link element called first.

  • Each Link carries a data field(s) and a Link Field called next.

  • Each Link is linked with its next link using its next link.

  • Last Link carries a Link as null to mark the end of the list.

Types of Linked List

Following are the various flavours of linked list.

  • Simple Linked List − Item Navigation is forward only.

  • Doubly Linked List − Items can be navigated forward and backward way.

  • Circular Linked List − Last item contains link of the first element as next and and first element has link to last element as prev.

Basic Operations

Following are the basic operations supported by a list.

  • Insertion − add an element at the beginning of the list.

  • Deletion − delete an element at the beginning of the list.

  • Display − displaying complete list.

  • Search − search an element using given key.

  • Delete − delete an element using given key.

Insertion Operation

Insertion is a three step process:

  1. Create a new Link with provided data.

  2. Point New Link to old First Link.

  3. Point First Link to this New Link.

Linked List Insert First 
//insert link at the first location
public void insertFirst(int key, int data){
   //create a link
   Link link = new Link(key,data);
   //point it to old first node
   link.next = first;
   //point first to new first node
   first = link;
}

Deletion Operation

Deletion is a two step process:

  1. Get the Link pointed by First Link as Temp Link.

  2. Point First Link to Temp Link's Next Link.

Linked List Delete First 
//delete first item
public Link deleteFirst(){
   //save reference to first link
   Link tempLink = first;
   //mark next to first link as first 
   first = first.next;
   //return the deleted link
   return tempLink;
}

Navigation Operation

Navigation is a recursive step process and is basis of many operations like search, delete etc.:

  1. Get the Link pointed by First Link as Current Link.

  2. Check if Current Link is not null and display it.

  3. Point Current Link to Next Link of Current Link and move to above step.

Linked List Navigation

Note 

//display the list
public void display(){
   //start from the beginning
   Link current = first;
   //navigate till the end of the list
   System.out.print("[ ");
   while(current != null){
      //print data
      current.display();
      //move to next item
      current = current.next;
      System.out.print(" ");
   }
   System.out.print(" ]");
}

Advanced Operations

Following are the advanced operations specified for a list.

  • Sort − sorting a list based on a particular order.

  • Reverse − reversing a linked list.

  • Concatenate − concatenate two lists.

Sort Operation

We've used bubble sort to sort a list.

public void sort(){

   int i, j, k, tempKey, tempData ;
   Link current,next;
   int size = length();
   k = size ;
   for ( i = 0 ; i < size - 1 ; i++, k-- ) {
      current = first ;
      next = first.next ;
      for ( j = 1 ; j < k ; j++ ) {            
         if ( current.data > next.data ) {
            tempData = current.data ;
            current.data = next.data;
            next.data = tempData ;

            tempKey = current.key;
            current.key = next.key;
            next.key = tempKey;
         }
         current = current.next;
         next = next.next;                        
      }
   }
}

Reverse Operation

Following code demonstrate reversing a single linked list.

public LinkedList reverse() { 
   LinkedList reversedlist = new LinkedList();
   Link nextLink = null;     
   reversedlist.insertFirst(first.key, first.data);

   Link currentLink = first;       
   // Until no more data in list, 
   // insert current link before first and move ahead.
   while(currentLink.next != null){
      nextLink = currentLink.next;           
      // Insert at start of new list.
      reversedlist.insertFirst(nextLink.key, nextLink.data); 
      //advance to next node
      currentLink = currentLink.next;            
   }      
   return reversedlist;
}

Concatenate Operation

Following code demonstrate reversing a single linked list.

public void concatenate(LinkedList list){
   if(first == null){
      first = list.first;
   }
   if(list.first == null){
      return;
   }
   Link temp = first;
   while(temp.next !=null) {
      temp = temp.next;
   }
   temp.next = list.first;       
}

Demo

Link.java 
package com.tutorialspoint.list;

public class Link {
   public int key;
   public int data;
   public Link next;

   public Link(int key, int data){
      this.key = key;
      this.data = data;
   }

   public void display(){
      System.out.print("{"+key+","+data+"}");
   }
}
LinkedList.java 
package com.tutorialspoint.list;

public class LinkedList {
   //this link always point to first Link 
   //in the Linked List 
   private Link first;

   // create an empty linked list 
   public LinkedList(){
      first = null;
   }

   //insert link at the first location
   public void insertFirst(int key, int data){
      //create a link
      Link link = new Link(key,data);
      //point it to old first node
      link.next = first;
      //point first to new first node
      first = link;
   }

   //delete first item
   public Link deleteFirst(){
      //save reference to first link
      Link tempLink = first;
      //mark next to first link as first 
      first = first.next;
      //return the deleted link
      return tempLink;
   }

   //display the list
   public void display(){
      //start from the beginning
      Link current = first;
      //navigate till the end of the list
      System.out.print("[ ");
      while(current != null){
         //print data
         current.display();
         //move to next item
         current = current.next;
         System.out.print(" ");
      }
      System.out.print(" ]");
   }

   //find a link with given key
   public Link find(int key){
      //start from the first link
      Link current = first;

      //if list is empty
      if(first == null){
         return null;
      }
      //navigate through list
      while(current.key != key){
         //if it is last node
         if(current.next == null){
            return null;
         }else{
            //go to next link
            current = current.next;
         }
      }      
      //if data found, return the current Link
      return current;
   }

   //delete a link with given key
   public Link delete(int key){
      //start from the first link
      Link current = first;
      Link previous = null;
      //if list is empty
      if(first == null){
         return null;
      }

      //navigate through list
      while(current.key != key){
         //if it is last node
         if(current.next == null){
            return null;
         }else{
            //store reference to current link
            previous = current;
            //move to next link
            current = current.next;             
         }
      }

      //found a match, update the link
      if(current == first) {
         //change first to point to next link
         first = first.next;
      }else {
         //bypass the current link
         previous.next = current.next;
      }    
      return current;
   }


   //is list empty
   public boolean isEmpty(){
      return first == null;
   }
   
   public int length(){
      int length = 0;
      for(Link current = first; current!=null;
         current = current.next){
         length++;
      }
      return length;
   }
   
   public void sort(){
      int i, j, k, tempKey, tempData ;
      Link current,next;
      int size = length();
      k = size ;
      for ( i = 0 ; i < size - 1 ; i++, k-- ) {
         current = first ;
         next = first.next ;
         for ( j = 1 ; j < k ; j++ ) {            
            if ( current.data > next.data ) {
               tempData = current.data ;
               current.data = next.data;
               next.data = tempData ;
	 
	           tempKey = current.key;
	           current.key = next.key;
	           next.key = tempKey;
            }
            current = current.next;
           next = next.next;                        
         }
      }
   }

   public LinkedList reverse() { 
      LinkedList reversedlist = new LinkedList();
      Link nextLink = null;     
      reversedlist.insertFirst(first.key, first.data);

      Link currentLink = first;       
      // Until no more data in list, 
      // insert current link before first and move ahead.
      while(currentLink.next != null){
         nextLink = currentLink.next;           
         // Insert at start of new list.
         reversedlist.insertFirst(nextLink.key, nextLink.data); 
         //advance to next node
         currentLink = currentLink.next;            
      }      
      return reversedlist;
   }

   public void concatenate(LinkedList list){
      if(first == null){
         first = list.first;
      }
      if(list.first == null){
         return;
      }
      Link temp = first;

      while(temp.next !=null) {
         temp = temp.next;
      }
      temp.next = list.first;       
   }
}
LinkedListDemo.java 
package com.tutorialspoint.list;

public class LinkedListDemo {
   public static void main(String args[]){
      LinkedList list = new LinkedList();
        
      list.insertFirst(1, 10);
      list.insertFirst(2, 20);
      list.insertFirst(3, 30);
      list.insertFirst(4, 1);
      list.insertFirst(5, 40);
      list.insertFirst(6, 56);

      System.out.print("\nOriginal List: ");  
      list.display();
      System.out.println("");
      while(!list.isEmpty()){            
         Link temp = list.deleteFirst();
         System.out.print("Deleted value:");  
         temp.display();
         System.out.println("");
      }         
      System.out.print("List after deleting all items: ");          
      list.display();
      System.out.println("");
      list.insertFirst(1, 10);
      list.insertFirst(2, 20);
      list.insertFirst(3, 30);
      list.insertFirst(4, 1);
      list.insertFirst(5, 40);
      list.insertFirst(6, 56);

      System.out.print("Restored List: ");  
      list.display();
      System.out.println("");  

      Link foundLink = list.find(4);
      if(foundLink != null){
        System.out.print("Element found: ");  
         foundLink.display();
         System.out.println("");  
      }else{
         System.out.println("Element not found.");  
      }

      list.delete(4);
      System.out.print("List after deleting an item: ");  
      list.display();
      System.out.println(""); 
      foundLink = list.find(4);
      if(foundLink != null){
         System.out.print("Element found: ");  
         foundLink.display();
         System.out.println("");  
      }else{
         System.out.print("Element not found. {4,1}");  
      }
      System.out.println(""); 
      list.sort();
      System.out.print("List after sorting the data: ");  
      list.display();
      System.out.println(""); 
      System.out.print("Reverse of the list: ");  
      LinkedList list1 = list.reverse();
      list1.display();
      System.out.println(""); 
      
      LinkedList list2 = new LinkedList();

      list2.insertFirst(9, 50);
      list2.insertFirst(8, 40);
      list2.insertFirst(7, 20);

      list.concatenate(list2);
      System.out.print("List after concatenation: ");  
      list.display();
      System.out.println(""); 
   }
}

If we compile and run the above program then it would produce following result:

Original List: [ {6,56} {5,40} {4,1} {3,30} {2,20} {1,10}  ]
Deleted value:{6,56}
Deleted value:{5,40}
Deleted value:{4,1}
Deleted value:{3,30}
Deleted value:{2,20}
Deleted value:{1,10}
List after deleting all items: [  ]
Restored List: [ {6,56} {5,40} {4,1} {3,30} {2,20} {1,10}  ]
Element found: {4,1}
List after deleting an item: [ {6,56} {5,40} {3,30} {2,20} {1,10}  ]
Element not found. {4,1}
List after sorting the data: [ {1,10} {2,20} {3,30} {5,40} {6,56}  ]
Reverse of the list: [ {6,56} {5,40} {3,30} {2,20} {1,10}  ]
List after concatenation: [ {1,10} {2,20} {3,30} {5,40} {6,56} {7,20} {8,40} {9,50}  ]
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