Precourse 1

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"); 
+    } 
+}

Exercise_2.cs

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+
+//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());
+    }
+}