Adding all the solutions

Exercise_1.java

@@ -5,31 +5,62 @@ class Stack {
     int top; 
     int a[] = new int[MAX]; // Maximum size of Stack 
 
+    // Time Complexity: O(1)
+    // Space Complexity: O(1)
     boolean isEmpty() 
     { 
         //Write your code here 
+        return top ==0;
     } 
 
     Stack() 
     { 
         //Initialize your constructor 
+        top = 0;
     } 
 
+    // Time Complexity: O(1)
+    // Space Complexity: O(1)
     boolean push(int x) 
     { 
         //Check for stack Overflow
         //Write your code here
+
+        if(top <= MAX) {
+            a[top] = x;
+            top++;
+            return true;
+        } else {
+            System.out.println("Stack Overflow");
+            return false;
+        }
+
     } 
 
+    // Time Complexity: O(1)
+    // Space Complexity: O(1)
     int pop() 
     { 
         //If empty return 0 and print " Stack Underflow"
         //Write your code here
+        if(isEmpty()) {
+           System.out.println("Stack Underflow"); 
+           return 0;
+        }
+
+        return a[--top];
     } 
 
+    // Time Complexity: O(1)
+    // Space Complexity: O(1)
     int peek() 
     { 
         //Write your code here
+        if (isEmpty()) {
+            System.out.println("Stack is Empty");
+            return 0;
+        }
+        return a[top-1];
     } 
 } 
 
@@ -39,8 +70,10 @@ public static void main(String args[])
     { 
         Stack s = new Stack(); 
         s.push(10); 
-        s.push(20); 
+        s.push(20);
+        s.pop();
         s.push(30); 
+        s.pop();
         System.out.println(s.pop() + " Popped from stack"); 
     } 
 }

Exercise_2.java

@@ -1,4 +1,4 @@
-public class StackAsLinkedList { 
+public class Exercise_2 { 
 
     StackNode root; 
 
@@ -9,37 +9,65 @@ static class StackNode {
         StackNode(int data) 
         { 
             //Constructor here 
+            this.data = data;
+            this.next = null;
         } 
     } 
 
-
+	// Time Complexity: O(1)
+    // Space Complexity: O(1)
     public boolean isEmpty() 
     { 
         //Write your code here for the condition if stack is empty. 
+        return root == null;
+
     } 
 
+    // Time Complexity: O(1)
+    // Space Complexity: O(1)
     public void push(int data) 
     { 
         //Write code to push data to the stack. 
+        StackNode newNode = new StackNode(data);
+        newNode.next = root;
+        root = newNode;
+
     } 
 
+    // Time Complexity: O(1)
+    // Space Complexity: O(1)
     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()) {
+        System.out.println("Stack Underflow");
+        return 0;
+    }
+    StackNode temp = root;
+    root = root.next;
+    temp.next = null;
+    return temp.data;
+
     } 
 
+    // Time Complexity: O(1)
+    // Space Complexity: O(1)
     public int peek() 
     { 
         //Write code to just return the topmost element without removing it.
+        if (isEmpty()) {
+            return 0;
+        }
+        return root.data;
     } 
 
 	//Driver code
     public static void main(String[] args) 
     { 
 
-        StackAsLinkedList sll = new StackAsLinkedList(); 
+        Exercise_2 sll = new Exercise_2(); 
 
         sll.push(10); 
         sll.push(20);