diff --git a/.DS_Store b/.DS_Store
new file mode 100644
index 000000000..5008ddfcf
Binary files /dev/null and b/.DS_Store differ
diff --git a/Exercise_1.cpp b/Exercise_1.cpp
deleted file mode 100644
index 381e274d5..000000000
--- a/Exercise_1.cpp
+++ /dev/null
@@ -1,54 +0,0 @@
-#include <bits/stdc++.h> 
-  
-using namespace std; 
-  
-#define MAX 1000 
-  
-class Stack {
-  //Please read sample.java file before starting.
-  //Kindly include Time and Space complexity at top of each file
-    int top; 
-  
-public: 
-    int a[MAX]; // Maximum size of Stack 
-  
-    Stack() { //Constructor here } 
-    bool push(int x); 
-    int pop(); 
-    int peek(); 
-    bool isEmpty(); 
-}; 
-  
-bool Stack::push(int x) 
-{ 
-    //Your code here
-    //Check Stack overflow as well
-} 
-  
-int Stack::pop() 
-{ 
-    //Your code here
-    //Check Stack Underflow as well 
-} 
-int Stack::peek() 
-{ 
-    //Your code here
-    //Check empty condition too
-} 
-  
-bool Stack::isEmpty() 
-{ 
-    //Your code here 
-} 
-  
-// Driver program to test above functions 
-int main() 
-{ 
-    class Stack s; 
-    s.push(10); 
-    s.push(20); 
-    s.push(30); 
-    cout << s.pop() << " Popped from stack\n"; 
-  
-    return 0; 
-} 
diff --git a/Exercise_1.cs b/Exercise_1.cs
new file mode 100644
index 000000000..8ae5ead3e
--- /dev/null
+++ b/Exercise_1.cs
@@ -0,0 +1,70 @@
+class Stack {
+ 
+    // Time Complexity Except Push is O(1) , Push O(n)
+    // Space Complexity = O(n)
+    const int MAX = 1000;
+    int top;
+    int[] arr = new int[MAX]; // Maximum size of Stack 
+
+    public bool isEmpty()
+    {
+        //Write your code here 
+        return top == -1;
+    }
+
+    public Stack()
+    {
+        //Initialize  constructor 
+        top = -1;
+    }
+
+    public void Resize()
+    {
+        int capacity = arr.Length * 2;
+        int[] newarr = new int[capacity];
+        Array.Copy(arr, newarr, arr.Length);
+        arr = newarr;
+        
+    }
+    public void push(int x)
+    {
+        if (top == arr.Length - 1)
+        {
+            Resize();
+        }
+        arr[++top] = x;
+       
+    }
+
+    public int pop()
+    {
+        //If empty return 0 and print " Stack Underflow"
+        if (isEmpty())
+        {
+            Console.Write(" Stack Underflow");
+            return 0;
+        }
+        return arr[top--];
+    }
+
+    public int peek()
+    {
+        if (isEmpty())
+        {
+            return 0;
+        }
+        return arr[top];
+    } 
+} 
+  
+// Driver code 
+class Program { 
+    public static void Main(String[] args) 
+    { 
+        Stack s = new Stack(); 
+        s.push(10); 
+        s.push(20); 
+        s.push(30); 
+        Console.Write(s.pop() + " Popped from stack"); 
+    } 
+}
diff --git a/Exercise_1.java b/Exercise_1.java
deleted file mode 100644
index 314a3cb45..000000000
--- a/Exercise_1.java
+++ /dev/null
@@ -1,46 +0,0 @@
-class Stack { 
-    //Please read sample.java file before starting.
-  //Kindly include Time and Space complexity at top of each file
-    static final int MAX = 1000; 
-    int top; 
-    int a[] = new int[MAX]; // Maximum size of Stack 
-  
-    boolean isEmpty() 
-    { 
-        //Write your code here 
-    } 
-
-    Stack() 
-    { 
-        //Initialize your constructor 
-    } 
-  
-    boolean push(int x) 
-    { 
-        //Check for stack Overflow
-        //Write your code here
-    } 
-  
-    int pop() 
-    { 
-        //If empty return 0 and print " Stack Underflow"
-        //Write your code here
-    } 
-  
-    int peek() 
-    { 
-        //Write your code here
-    } 
-} 
-  
-// Driver code 
-class Main { 
-    public static void main(String args[]) 
-    { 
-        Stack s = new Stack(); 
-        s.push(10); 
-        s.push(20); 
-        s.push(30); 
-        System.out.println(s.pop() + " Popped from stack"); 
-    } 
-}
diff --git a/Exercise_1.js b/Exercise_1.js
deleted file mode 100644
index 207189ea0..000000000
--- a/Exercise_1.js
+++ /dev/null
@@ -1,35 +0,0 @@
-class Stack {
-  //Please read sample.java file before starting.
-  //Kindly include Time and Space complexity at top of each file
-​
-    constructor() {
-        //Initialize your constructor
-        this.MAX = 1000;
-        this.top = -1;
-        this.a = new Array(this.MAX);
-    }
-​
-    function isEmpty() {
-        //Write your code here
-    }
-​
-    function push(x) {
-        //Check for stack Overflow
-        //Write your code here
-    }
-​
-    function pop() {
-        //If empty return 0 and print " Stack Underflow"
-        //Write your code here
-    }
-​
-    function peek() {
-       //Write your code here
-    }
-}
-​
-let s = new Stack();
-s.push(10);
-s.push(20);
-s.push(30);
-console.log(s.pop() + " Popped from stack");
diff --git a/Exercise_1.py b/Exercise_1.py
deleted file mode 100644
index 532833f5d..000000000
--- a/Exercise_1.py
+++ /dev/null
@@ -1,24 +0,0 @@
-class myStack:
-  #Please read sample.java file before starting.
-  #Kindly include Time and Space complexity at top of each file
-     def __init__(self):
-         
-     def isEmpty(self):
-         
-     def push(self, item):
-         
-     def pop(self):
-        
-        
-     def peek(self):
-        
-     def size(self):
-         
-     def show(self):
-         
-
-s = myStack()
-s.push('1')
-s.push('2')
-print(s.pop())
-print(s.show())
diff --git a/Exercise_2.cpp b/Exercise_2.cpp
deleted file mode 100644
index 1eb3de9b9..000000000
--- a/Exercise_2.cpp
+++ /dev/null
@@ -1,52 +0,0 @@
-#include <bits/stdc++.h> 
-using namespace std; 
-  
-// A structure to represent a stack 
-class StackNode { 
-public: 
-    int data; 
-    StackNode* next; 
-}; 
-  
-StackNode* newNode(int data) 
-{ 
-    StackNode* stackNode = new StackNode(); 
-    stackNode->data = data; 
-    stackNode->next = NULL; 
-    return stackNode; 
-} 
-  
-int isEmpty(StackNode* root) 
-{ 
-    //Your code here 
-} 
-  
-void push(StackNode** root, int data) 
-{ 
-    //Your code here 
-} 
-  
-int pop(StackNode** root) 
-{ 
-    //Your code here 
-} 
-  
-int peek(StackNode* root) 
-{ 
-    //Your code here 
-} 
-  
-int main() 
-{ 
-    StackNode* root = NULL; 
-  
-    push(&root, 10); 
-    push(&root, 20); 
-    push(&root, 30); 
-  
-    cout << pop(&root) << " popped from stack\n"; 
-  
-    cout << "Top element is " << peek(root) << endl; 
-  
-    return 0; 
-} 
\ No newline at end of file
diff --git a/Exercise_2.cs b/Exercise_2.cs
new file mode 100644
index 000000000..580b5224f
--- /dev/null
+++ b/Exercise_2.cs
@@ -0,0 +1,75 @@
+
+//Time Complexity - O(1)
+//Space Complexity - O(n)
+public class StackAsLinkedList
+{
+
+    StackNode root;
+
+    private class StackNode
+    {
+        public int data;
+        public StackNode next;
+
+        public StackNode(int data)
+        {
+            //Constructor here 
+            this.data = data;
+            next = null;
+        }
+    }
+
+
+    public bool isEmpty()
+    {
+        return root == null;
+    }
+
+    public void push(int data)
+    {
+        //Write code to push data to the stack. 
+        StackNode st = new StackNode(data);
+        st.next = root;
+        root = st;
+    }
+
+    public int pop()
+    {
+        //If Stack Empty Return 0 and print "Stack Underflow"
+        //Write code to pop the topmost element of stack.
+        //Also return the popped element 
+        if (isEmpty())
+        {
+            Console.WriteLine("Stack Underflow");
+            return 0;
+        }
+        int popped = root.data;
+        root = root.next;
+        return popped;
+    }
+
+    public int peek()
+    {
+        //Write code to just return the topmost element without removing it.
+        if (isEmpty())
+        {
+            Console.WriteLine("Stack Underflow");
+            return 0;
+        }
+        return root.data;
+    }
+
+    //Driver code
+    public static void Main(String[] args)
+    {
+        StackAsLinkedList sll = new StackAsLinkedList();
+
+        sll.push(10);
+        sll.push(20);
+        sll.push(30);
+
+        Console.Write(sll.pop() + " popped from stack");
+
+        Console.Write("Top element is " + sll.peek());
+    }
+} 
diff --git a/Exercise_2.java b/Exercise_2.java
deleted file mode 100644
index 5a9c4868c..000000000
--- a/Exercise_2.java
+++ /dev/null
@@ -1,52 +0,0 @@
-public class StackAsLinkedList { 
-  
-    StackNode root; 
-  
-    static class StackNode { 
-        int data; 
-        StackNode next; 
-  
-        StackNode(int data) 
-        { 
-            //Constructor here 
-        } 
-    } 
-    
-	
-    public boolean isEmpty() 
-    { 
-        //Write your code here for the condition if stack is empty. 
-    } 
-  
-    public void push(int data) 
-    { 
-        //Write code to push data to the stack. 
-    } 
-  
-    public int pop() 
-    { 	
-	//If Stack Empty Return 0 and print "Stack Underflow"
-        //Write code to pop the topmost element of stack.
-	//Also return the popped element 
-    } 
-  
-    public int peek() 
-    { 
-        //Write code to just return the topmost element without removing it.
-    } 
-  
-	//Driver code
-    public static void main(String[] args) 
-    { 
-  
-        StackAsLinkedList sll = new StackAsLinkedList(); 
-  
-        sll.push(10); 
-        sll.push(20); 
-        sll.push(30); 
-  
-        System.out.println(sll.pop() + " popped from stack"); 
-  
-        System.out.println("Top element is " + sll.peek()); 
-    } 
-} 
diff --git a/Exercise_2.js b/Exercise_2.js
deleted file mode 100644
index 2e3216f94..000000000
--- a/Exercise_2.js
+++ /dev/null
@@ -1,36 +0,0 @@
-class StackAsLinkedList {
-​
-    static stackNode = class {
-​
-        constructor(d) {
-            //Constructor here
-            this.data = d;
-            this.next = null;
-        }
-    }
-​
-    function isEmpty() {
-        //Write your code here for the condition if stack is empty.
-    }
-​
-    function push(data) {
-       //Write code to push data to the stack.
-    }
-​
-    function pop() {
-       //If Stack Empty Return 0 and print "Stack Underflow"
-       //Write code to pop the topmost element of stack.
-       //Also return the popped element
-    }
-​
-    function peek() {
-       //Write code to just return the topmost element without removing it.
-    }
-}
-//Driver code
-const sll = new StackAsLinkedList();
-sll.push(10);
-sll.push(20);
-sll.push(30);
-console.log(sll.pop() + " popped from stack");
-console.log("Top element is " + sll.peek());
diff --git a/Exercise_2.py b/Exercise_2.py
deleted file mode 100644
index b11492215..000000000
--- a/Exercise_2.py
+++ /dev/null
@@ -1,32 +0,0 @@
-
-class Node:
-    def __init__(self, data):
-       self.data = data
-       self.next = None
- 
-class Stack:
-    def __init__(self):
-        
-    def push(self, data):
-        
-    def pop(self):
-        
-a_stack = Stack()
-while True:
-    #Give input as string if getting an EOF error. Give input like "push 10" or "pop"
-    print('push <value>')
-    print('pop')
-    print('quit')
-    do = input('What would you like to do? ').split()
-    #Give input as string if getting an EOF error. Give input like "push 10" or "pop"
-    operation = do[0].strip().lower()
-    if operation == 'push':
-        a_stack.push(int(do[1]))
-    elif operation == 'pop':
-        popped = a_stack.pop()
-        if popped is None:
-            print('Stack is empty.')
-        else:
-            print('Popped value: ', int(popped))
-    elif operation == 'quit':
-        break
diff --git a/Exercise_3.cpp b/Exercise_3.cpp
deleted file mode 100644
index f34d89ac1..000000000
--- a/Exercise_3.cpp
+++ /dev/null
@@ -1,80 +0,0 @@
-#include <bits/stdc++.h> 
-using namespace std; 
-  
-// A linked list node (changes) 
-class Node  
-{  
-    public: 
-    int data;  
-    Node *next;  
-};  
-  
-/* Given a reference (pointer to pointer) 
-to the head of a list and an int, inserts 
-a new node on the front of the list. */
-void push(Node** head_ref, int new_data)  
-{  
-    /* 1. allocate node */ 
-  
-    /* 2. put in the data */  
-  
-    /* 3. Make next of new node as head */ 
-  
-    /* 4. move the head to point to the new node */
-}  
-  
-/* Given a node prev_node, insert a new node after the given  
-prev_node */
-void insertAfter(Node* prev_node, int new_data)  
-{  
-    /*1. check if the given prev_node is NULL */ 
-  
-    /* 2. allocate new node */ 
-  
-    /* 3. put in the data */ 
-  
-    /* 4. Make next of new node as next of prev_node */
-  
-    /* 5. move the next of prev_node as new_node */ 
-}  
-  
-/* Given a reference (pointer to pointer) to the head  
-of a list and an int, appends a new node at the end */
-void append(Node** head_ref, int new_data)  
-{  
-    /* 1. allocate node */ 
-  
-    /* 2. put in the data */ 
-  
-    /* 3. This new node is going to be  
-    the last node, so make next of  
-    it as NULL*/  
-  
-    /* 4. If the Linked List is empty, 
-    then make the new node as head */
-  
-    /* 5. Else traverse till the last node */
-  
-    /* 6. Change the next of last node */ 
-}  
-  
-// This function prints contents of 
-// linked list starting from head  
-void printList(Node *node)  
-{  
-    //Your code here
-}  
-  
-/* Driver code*/
-int main()  
-{  
-    Node* head = NULL;   
-    append(&head, 6);  
-    push(&head, 7);  
-    push(&head, 1);    
-    append(&head, 4);    
-    insertAfter(head->next, 8);  
-    cout<<"Created Linked list is: ";  
-    printList(head);  
-    return 0;  
-}  
\ No newline at end of file
diff --git a/Exercise_3.cs b/Exercise_3.cs
new file mode 100644
index 000000000..c314632c4
--- /dev/null
+++ b/Exercise_3.cs
@@ -0,0 +1,90 @@
+
+//Time Complexity - O(n)
+//Space Complexity - O(n)
+public class LinkedList { 
+  
+    Node head; // head of list 
+  
+    // Linked list Node. 
+    // This inner class is made static 
+    // so that main() can access it 
+    static class Node { 
+  
+        int data; 
+        public Node next;
+
+        // Constructor 
+        public Node(int d)
+        {
+            //Write your code here 
+            data = d;
+            next = null;
+        } 
+    }
+
+    // Method to insert a new node 
+    public static LinkedList insert(LinkedList list, int data)
+    {
+        // Create a new node with given data 
+        Node newNode = new Node(data);
+
+        // If the Linked List is empty, 
+        // then make the new node as head 
+
+        if (list.head == null)
+        {
+            list.head = newNode;
+        }
+        // Else traverse till the last node 
+        // and insert the new_node there 
+        else
+        {
+            Node cur = list.head;
+            while (cur.next != null)
+            {
+                cur = cur.next;
+            }
+            cur.next = newNode;
+        }
+        // Insert the new_node at last node 
+        // Return the list by head 
+        return list;
+    }
+
+    // Method to print the LinkedList. 
+    public static void printList(LinkedList list)
+    {
+        // Traverse through the LinkedList 
+
+        // Print the data at current node 
+
+        // Go to next node 
+        Node cur = list.head;
+        while (cur != null)
+        {
+            Console.WriteLine(cur.data);
+            cur = cur.next;
+        }
+    } 
+   
+    // Driver code 
+    public static void main(String[] args) 
+    { 
+        /* Start with the empty list. */
+        LinkedList list = new LinkedList(); 
+  
+        // 
+        // ******INSERTION****** 
+        // 
+  
+        // Insert the values 
+        list = insert(list, 1); 
+        list = insert(list, 2); 
+        list = insert(list, 3); 
+        list = insert(list, 4); 
+        list = insert(list, 5); 
+  
+        // Print the LinkedList 
+        printList(list); 
+    } 
+}
\ No newline at end of file
diff --git a/Exercise_3.java b/Exercise_3.java
deleted file mode 100644
index fb66d329d..000000000
--- a/Exercise_3.java
+++ /dev/null
@@ -1,70 +0,0 @@
-import java.io.*; 
-  
-// Java program to implement 
-// a Singly Linked List 
-public class LinkedList { 
-  
-    Node head; // head of list 
-  
-    // Linked list Node. 
-    // This inner class is made static 
-    // so that main() can access it 
-    static class Node { 
-  
-        int data; 
-        Node next; 
-  
-        // Constructor 
-        Node(int d) 
-        { 
-            //Write your code here 
-        } 
-    } 
-  
-    // Method to insert a new node 
-    public static LinkedList insert(LinkedList list, int data) 
-    { 
-        // Create a new node with given data 
-   
-        // If the Linked List is empty, 
-        // then make the new node as head 
-        
-            // Else traverse till the last node 
-            // and insert the new_node there 
-
-            // Insert the new_node at last node 
-        // Return the list by head 
-        
-    } 
-  
-    // Method to print the LinkedList. 
-    public static void printList(LinkedList list) 
-    {  
-        // Traverse through the LinkedList 
-   
-            // Print the data at current node 
-       
-            // Go to next node 
-    } 
-   
-    // Driver code 
-    public static void main(String[] args) 
-    { 
-        /* Start with the empty list. */
-        LinkedList list = new LinkedList(); 
-  
-        // 
-        // ******INSERTION****** 
-        // 
-  
-        // Insert the values 
-        list = insert(list, 1); 
-        list = insert(list, 2); 
-        list = insert(list, 3); 
-        list = insert(list, 4); 
-        list = insert(list, 5); 
-  
-        // Print the LinkedList 
-        printList(list); 
-    } 
-}
\ No newline at end of file
diff --git a/Exercise_3.js b/Exercise_3.js
deleted file mode 100644
index d1511f80e..000000000
--- a/Exercise_3.js
+++ /dev/null
@@ -1,49 +0,0 @@
-// Java program to implement
-// a Singly Linked List
-class LinkedList {
-    constructor() {
-        this.head = null;
-    }
-     // Linked list Node.
-     static Node = class {
-        constructor(d) {
-            this.data = d;
-            this.next = null;
-        }
-    }
-​
-     // Method to insert a new node
-    function insert(list, data) {
-        // Create a new node with given data
-​
-        // If the Linked List is empty,
-        // then make the new node as head
-​
-        // Else traverse till the last node
-        // and insert the new_node there
-​
-        // Insert the new_node at last node
-        // Return the list by head
-    }
-​
-     // Method to print the LinkedList.
-    function printList(list) {
-        // Traverse through the LinkedList
-​
-        // Print the data at current node
-​
-        // Go to next node
-    }
-}
-       // Driver code
-       /* Start with the empty list. */
-       let list = new LinkedList();
-​
-        // ******INSERTION******
-        // Insert the values
-        list.insert(list, 1);
-        list.insert(list, 2);
-        list.insert(list, 3);
-        list.insert(list, 4);
-        // Print the LinkedList
-        list.printList(list);
diff --git a/Exercise_3.py b/Exercise_3.py
deleted file mode 100644
index a5d466b59..000000000
--- a/Exercise_3.py
+++ /dev/null
@@ -1,32 +0,0 @@
-class ListNode:
-    """
-    A node in a singly-linked list.
-    """
-    def __init__(self, data=None, next=None):
-    
-class SinglyLinkedList:
-    def __init__(self):
-        """
-        Create a new singly-linked list.
-        Takes O(1) time.
-        """
-        self.head = None
-
-    def append(self, data):
-        """
-        Insert a new element at the end of the list.
-        Takes O(n) time.
-        """
-        
-    def find(self, key):
-        """
-        Search for the first element with `data` matching
-        `key`. Return the element or `None` if not found.
-        Takes O(n) time.
-        """
-        
-    def remove(self, key):
-        """
-        Remove the first occurrence of `key` in the list.
-        Takes O(n) time.
-        """
diff --git a/PreCourse-1 b/PreCourse-1
new file mode 160000
index 000000000..45e4a7270
--- /dev/null
+++ b/PreCourse-1
@@ -0,0 +1 @@
+Subproject commit 45e4a7270718f96153bfd32826a98148ad70efdb