completed PreCoruse-1

Exercise_1.java

@@ -1,35 +1,53 @@
 class Stack { 
-    //Please read sample.java file before starting.
-  //Kindly include Time and Space complexity at top of each file
+    // Time Complexcity : 
+    // push() - o(1)
+    // pop() - o(1)
+    // peek() - o(1)
+    // isEmpty() - o(1)
+    // Space Complexity : o(MAX)
+
     static final int MAX = 1000; 
     int top; 
     int a[] = new int[MAX]; // Maximum size of Stack 
 
     boolean isEmpty() 
-    { 
-        //Write your code here 
+    {
+        return (top < 0);
+
     } 
 
     Stack() 
     { 
-        //Initialize your constructor 
+       top = -1 ;
     } 
 
     boolean push(int x) 
     { 
-        //Check for stack Overflow
-        //Write your code here
-    } 
+        if (top >= (MAX - 1)){
+            System.out.println("Stack Overflow");
+            return false;
+        }
+            a[++top] = x;
+            return true;
+    }
 
     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--];
     } 
-
+
+
     int peek() 
     { 
-        //Write your code here
+        if (isEmpty()){
+            System.out.println("Stack is Empty");
+            return 0;
+        }
+            return a[top];
     } 
 } 
 

Exercise_3.java → LinkedList.java

@@ -1,6 +1,3 @@
-import java.io.*; 
-
-// Java program to implement 
 // a Singly Linked List 
 public class LinkedList { 
 
@@ -17,36 +14,45 @@ static class Node {
         // Constructor 
         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 new_node = new Node(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 
+        if (list.head == null){
+            list.head = new_node;
+            return list;
+        }
+        Node current = list.head;
+        while (current.next != null) {
+            current = current.next;
+        }
+        current.next = new_node;
+        return list;
+    }
 
-            // 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 
-
+        Node current = list.head;
+        while (current != null) {
             // Print the data at current node 
-
+            System.out.print(current.data + " ");
             // Go to next node 
-    } 
-
+            current = current.next;
+        }
+    }
+
     // Driver code 
     public static void main(String[] args) 
     { 

Exercise_2.java → StackAsLinkedList.java

@@ -8,33 +8,45 @@ static class StackNode {
 
         StackNode(int data) 
         { 
-            //Constructor here 
+            this.data = data;
+            this.next = null;
         } 
     } 
 
 
     public boolean isEmpty() 
     { 
-        //Write your code here for the condition if stack is empty. 
+        return (root == null);
     } 
 
     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 (isEmpty()){
+            System.out.println("Stack is Empty");
+            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()){
+            System.out.println("Stack is Empty");
+            return 0;
+        }
+        return root.data;
+    }
+
 	//Driver code
     public static void main(String[] args) 
     {