From a56dc240bead7ed575b53b2c92e4bf1cb70d5369 Mon Sep 17 00:00:00 2001
From: Rian Gallagher <rianogallagher@gmail.com>
Date: Mon, 28 Nov 2016 22:49:28 +0000
Subject: [PATCH] Added doubly linked list

---
 data_structures/DoublyLinkedList.java         | 141 ++++++++++++++++++
 ...LinkedLists.java => SinglyLinkedList.java} |   2 +-
 2 files changed, 142 insertions(+), 1 deletion(-)
 create mode 100644 data_structures/DoublyLinkedList.java
 rename data_structures/{LinkedLists.java => SinglyLinkedList.java} (98%)

diff --git a/data_structures/DoublyLinkedList.java b/data_structures/DoublyLinkedList.java
new file mode 100644
index 000000000..f8411994f
--- /dev/null
+++ b/data_structures/DoublyLinkedList.java
@@ -0,0 +1,141 @@
+/*
+ * A linked list is similar to an array, it holds values. However, links in a linked list do not have indexes.
+ * With a linked list you do not need to predetermine it's size as it grows and shrinks as it is edited.
+ * This is an example of a double ended, doubly linked list.
+ * Each link references the next link and the previous one.
+*/
+class LinkedList{
+	private Link head; //Head refers to the front of the list
+	private Link tail; //Tail refers to the back of the list
+
+	public LinkedList(){
+		head = null;
+		tail = null;
+	}
+
+	public void insertHead(int x){ //Insert an element at the head
+		Link newLink = new Link(x); //Create a new link with a value attached to it
+		if(isEmpty()) //Set the first element added to be the tail
+			tail = newLink;
+		else
+			head.previous = newLink; // newLink <-- currenthead(head)
+		newLink.next = head; // newLink <--> currenthead(head)
+		head = newLink; // newLink(head) <--> oldhead
+	}
+
+	public void insertTail(int x){
+		Link newLink = new Link(x);
+		newLink.next = null; // currentTail(tail)     newlink -->
+		tail.next = newLink; // currentTail(tail) --> newLink -->
+		newLink.previous = tail; // currentTail(tail) <--> newLink -->
+		tail = newLink; // oldTail <--> newLink(tail) -->
+	}
+
+	public Link deleteHead(){ //Delete the element at the head
+		Link temp = head;
+		head = head.next; // oldHead <--> 2ndElement(head)
+		head.previous = null; // oldHead --> 2ndElement(head) nothing pointing at old head so will be removed
+		if(head == null)
+			tail = null;
+		return temp;
+	}
+
+	public Link deleteTail(){
+		Link temp = tail;
+		tail = tail.previous; // 2ndLast(tail) <--> oldTail --> null
+ 		tail.next = null; // 2ndLast(tail) --> null
+		return temp;
+	}
+
+	public Link delete(int x){
+		Link current = head;
+
+		while(current.value != x) //Find the position to delete
+			current = current.next;
+
+		if(current == head)
+			deleteHead();
+
+		else if(current == tail)
+			deleteTail();
+
+		else{ //Before: 1 <--> 2(current) <--> 3
+			current.previous.next = current.next;  // 1 --> 3
+			current.next.previous = current.previous; // 1 <--> 3
+		}
+		return current;
+	}
+
+	public void insertOrdered(int x){
+		Link newLink = new Link(x);
+		Link current = head;
+		while(current != null && x > current.value) //Find the position to insert
+			current = current.next;
+
+		if(current == head)
+			insertHead(x);
+
+		else if(current == null)
+			insertTail(x);
+
+		else{ //Before: 1 <--> 2(current) <--> 3
+			newLink.previous = current.previous; // 1 <-- newLink
+			current.previous.next = newLink; // 1 <--> newLink
+			newLink.next = current; // 1 <--> newLink --> 2(current) <--> 3
+			current.previous = newLink; // 1 <--> newLink <--> 2(current) <--> 3
+		}
+	}
+
+	public boolean isEmpty(){ //Returns true if list is empty
+		return(head == null);
+	}
+
+	public void display(){ //Prints contents of the list
+		Link current = head;
+		while(current!=null){
+			current.displayLink();
+			current = current.next;
+		}
+		System.out.println();
+	}
+}
+
+class Link{
+	public int value;
+	public Link next; //This points to the link in front of the new link
+	public Link previous; //This points to the link behind the new link
+
+	public Link(int value){
+		this.value = value;
+	}
+
+	public void displayLink(){
+		System.out.print(value+" ");
+	}
+}
+
+//Example
+public class DoublyLinkedList{
+	public static void main(String args[]){
+		LinkedList myList = new LinkedList();
+
+		myList.insertHead(13);
+		myList.insertHead(7);
+		myList.insertHead(10);
+		myList.display(); // <-- 10(head) <--> 7 <--> 13(tail) -->
+
+		myList.insertTail(11);
+		myList.display(); // <-- 10(head) <--> 7 <--> 13 <--> 11(tail) -->
+
+		myList.deleteTail();
+		myList.display(); // <-- 10(head) <--> 7 <--> 13(tail) -->
+
+		myList.delete(7);
+		myList.display(); // <-- 10(head) <--> 13(tail) -->
+
+		myList.insertOrdered(23);
+		myList.insertOrdered(67);
+		myList.insertOrdered(3);
+		myList.display(); // <-- 3(head) <--> 10 <--> 13 <--> 23 <--> 67(tail) -->
+	}
+}
\ No newline at end of file
diff --git a/data_structures/LinkedLists.java b/data_structures/SinglyLinkedList.java
similarity index 98%
rename from data_structures/LinkedLists.java
rename to data_structures/SinglyLinkedList.java
index 1f1d32301..75660c731 100644
--- a/data_structures/LinkedLists.java
+++ b/data_structures/SinglyLinkedList.java
@@ -50,7 +50,7 @@ class Link{
 }
 
 //Example
-public class LinkedLists{
+public class SinglyLinkedList{
 	public static void main(String args[]){
 		LinkedList myList = new LinkedList();