added singly linked list in java and javascript (#53)

* added singly linked list in java

* added commnets

* addded singly linked list in javascript

* Rename SinglyLinkedLIst.java to singly-linked-list.java

linked-lists/java/singly-linked-list.java

@@ -0,0 +1,64 @@
+class Node<T> {
+    T data;
+    Node<T> next;
+
+    Node(T data) {
+        this.data = data;
+        this.next = null;
+    }
+}
+
+public class SinglyLinkedList<T> {
+    Node<T> head;
+
+    SinglyLinkedList() {
+        this.head = null;
+    }
+
+    void insertAtHead(T data) {
+        Node<T> newNode = new Node<T>(data);
+        if(this.head == null) {
+            this.head = newNode;
+        } else {
+            newNode.next = this.head;
+            this.head = newNode;
+        }
+    }
+
+    T removeAtHead() {
+        // check for underflow
+        if(this.head == null) {
+            System.out.println("Underflow");
+            System.exit(1);
+        }
+        // store data for return
+        T data = this.head.data;
+        this.head = this.head.next;
+        return data;
+    }
+
+    void printList() {
+        // if head is null then list is empty 
+        if(this.head == null) {
+            System.out.println("List is Empty");
+        } else {
+            // take itr same as head reference and loop through it until it became null
+            Node<T> itr = this.head;
+            while(itr != null) {
+                System.out.print(itr.data + " ");
+                itr = itr.next;
+            }
+            System.out.println("");
+        }
+    }
+
+    // Test Code
+    public static void main(String... args) {
+        SinglyLinkedList<String> l = new SinglyLinkedList<String>();
+        l.insertAtHead("abc");
+        l.insertAtHead("pqr");
+        l.insertAtHead("xyz");
+        l.removeAtHead();
+        l.printList();
+    }
+}

linked-lists/js/singly-linked-list.js

@@ -0,0 +1,55 @@
+class Node {
+    constructor(data) {
+        this.data = data;
+        this.next = null;
+    }
+}
+
+class SinglyLinkedList {
+    constructor() {
+        this.head = null;
+    }
+
+    insertAtHead(data) {
+        let newNode = new Node(data);
+        if(this.head == null) {
+            this.head = newNode;
+        } else {
+            newNode.next = this.head;
+            this.head = newNode;
+        }
+    }
+
+    removeAtHead() {
+        // check for underflow
+        if(this.head == null) {
+            console.log("Underflow");
+            return;
+        }
+        // store head's reference data value in local data variable
+        let data = this.head.data;
+        this.head = this.head.next;
+        return data;
+    }
+
+    printList() {
+        // if head is null then list is empty
+        if(this.head == null) {
+            console.log("List is Empty");
+        } else {
+            // iterate through list until itr is not null
+            let itr = this.head;
+            while(itr != null) {
+                console.log(itr.data);
+                itr = itr.next;
+            }
+        }
+    }
+}
+
+let l = new SinglyLinkedList();
+l.insertAtHead(1);
+l.insertAtHead('xyz');
+l.insertAtHead(1.1);
+l.removeAtHead();
+l.printList()