Completed pre-course 1

Exercise_1.java

@@ -1,46 +1,88 @@
-class Stack { 
+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) 
-    { 
+    //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()
+    {
+        // Time Complexity : O(1)
+        // Space Complexity : O(1)
+
+        //Write your code here
+        if(a.length == 0 || a == null){
+            return true;
+        }else{
+            return false;
+        }
+    }
+
+    Stack()
+    {
+        //Initialize your constructor
+        top = -1;
+    }
+
+
+    boolean push(int x)
+    {
+        // Time Complexity : O(1)
+        // Space Complexity : O(1)
+
         //Check for stack Overflow
+        if(top < MAX - 1){
+            top += 1;
+            a[top]= x;
+            return true;
+        }else{
+            System.out.println("***** Stack Overflow *****");
+            return false;
+        }
         //Write your code here
-    } 
-
-    int pop() 
-    { 
+    }
+
+    int pop()
+    {
+        // Time Complexity : O(1)
+        // Space Complexity : O(1)
+
         //If empty return 0 and print " Stack Underflow"
+        if(top < 0){
+            System.out.print("***** Stack Underflow : ");
+            return 0;
+        }else{
+            top -= 1;
+            return a[top+1];
+        }
         //Write your code here
-    } 
-
-    int peek() 
-    { 
+    }
+
+    int peek()
+    {
+        // Time Complexity : O(1)
+        // Space Complexity : O(1)
+
         //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"); 
-    } 
+        if(top == -1){
+            return 0;
+        }else{
+            return a[top];
+        }
+    }
+}
+
+// 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");
+        System.out.println(s.pop() + " Popped from stack");
+        System.out.println(s.pop() + " Popped from stack");
+        System.out.println(s.pop() + " Popped from stack");
+    }
 }

Exercise_2.java

@@ -1,52 +1,87 @@
-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"
+class StackAsLinkedList {
+
+    StackNode root;
+
+    static class StackNode {
+        int data;
+        StackNode next;
+
+        StackNode(int data) {
+            //Constructor here
+            this.data = data;
+            this.next = null;
+        }
+    }
+
+
+    public boolean isEmpty() {
+        // Time Complexity : O(1)
+        // Space Complexity : O(1)
+
+        if (root == null) {
+            return true;
+        } else {
+            return false;
+        }
+        //Write your code here for the condition if stack is empty.
+    }
+
+    public void push(int data) {
+        // Time Complexity : O(1)
+        // Space Complexity : O(1)
+
+        //Write code to push data to the stack.
+        if (root == null) {
+            root = new StackNode(data);
+            return;
+        }
+        StackNode node = new StackNode(data);
+        node.next = root;
+        root = node;
+
+    }
+
+    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() 
-    { 
+        //Also return the popped element
+
+        // Time Complexity : O(1)
+        // Space Complexity : O(1)
+
+        if (root == null) {
+            System.out.print("***** 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.
-    } 
-
-	//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()); 
-    } 
-} 
+        if (root == null) {
+            return 0;
+        }
+        return root.data;
+    }
+}
+class Main {
+    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(sll.pop() + " popped from stack");
+        System.out.println(sll.pop() + " popped from stack");
+        System.out.println(sll.pop() + " popped from stack");
+
+
+        System.out.println("Top element is " + sll.peek());
+    }
+}