exercise 1 done

Exercise_1.java

@@ -1,35 +1,71 @@
 class Stack { 
     //Please read sample.java file before starting.
   //Kindly include Time and Space complexity at top of each file
+// Time Complexity for push operation: O(1) 
+
+ // Time Complexity for pop operation : O(1)
+
+ //Time Complexity for peek operation : O(1)
+
+ //Space Complexity is O(n) since it has a constant space to store  n elements
+
+ // Its difficult to understamd how to start working on the problem but once I got idea its easy to implement
+
     static final int MAX = 1000; 
-    int top; 
-    int a[] = new int[MAX]; // Maximum size of Stack 
+    int top = -1; 
+    int[] a;
+
 
     boolean isEmpty() 
     { 
-        //Write your code here 
+        if(top == -1) {
+            return true;
+
+        }
+        return false;
+
     } 
 
     Stack() 
     { 
-        //Initialize your constructor 
+       a = new int[MAX];
     } 
 
     boolean push(int x) 
     { 
         //Check for stack Overflow
         //Write your code here
+
+      if(top == MAX-1){
+           throw new Error("Stack overflow exception");
+       }
+       a[top +1 ] = x;
+       top++;
+       return true;
     } 
 
     int pop() 
     { 
         //If empty return 0 and print " Stack Underflow"
         //Write your code here
+        if(top == -1){
+            System.out.println("Stack Underflow");
+            return 0;
+
+        }
+
+        return a[top--];
+
+
     } 
 
     int peek() 
     { 
-        //Write your code here
+
+    if(top == -1){
+        throw new Error("Empty Stack Exception");
+    }
+        return a[top];
     } 
 } 
 

Exercise_2.java

@@ -8,31 +8,50 @@ static class StackNode {
 
         StackNode(int data) 
         { 
-            //Constructor here 
+            this.data = data;
+
+
         } 
     } 
 
 
-    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. 
+
+        StackNode newNode = new StackNode(data);
+        newNode.next = root;
+        root = newNode;
     } 
 
     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(root == null) {
+            System.out.println("Stack Underflow");
+            return 0;
+        }
+
+        int returnData = root.data;
+
+        root = root.next;
+        return returnData;
+
     } 
 
     public int peek() 
     { 
         //Write code to just return the topmost element without removing it.
+
+        if(root != null){
+            return root.data;
+        }
+        return -1;
     } 
 
 	//Driver code