PreCourse-1 done

Exercise_1.java

@@ -1,3 +1,7 @@
+/**
+ * Time Complexity : O(1) for all operations (push, pop, peek, isEmpty)
+ * Space Complexity : O(N) where N is maximum size of the array (MAX)
+ */
 class Stack { 
     //Please read sample.java file before starting.
   //Kindly include Time and Space complexity at top of each file
@@ -8,28 +12,51 @@ class Stack {
     boolean isEmpty() 
     { 
         //Write your code here 
+        // A stack is empty if top is at its intial value
+        return top<0;
     } 
 
     Stack() 
     { 
         //Initialize your constructor 
+        this.top = -1;
     } 
 
     boolean push(int x) 
     { 
         //Check for stack Overflow
         //Write your code here
+        if(top>=MAX){
+            System.out.println("Stack Overflow");
+            return false;
+        }
+        else{
+            a[++top] = x;
+            return true;
+        }
     } 
 
     int pop() 
     { 
         //If empty return 0 and print " Stack Underflow"
         //Write your code here
+        if(top==-1)  {
+            System.out.print("Stack Underflow");
+            return 0;
+        }
+        int element = a[top--];
+        return element;
     } 
 
     int peek() 
     { 
         //Write your code here
+        if(top==-1){
+            System.out.println("Stack is Empty");
+            return 0;
+        }
+        else 
+          return a[top];
     } 
 } 
 

Exercise_2.java

@@ -1,38 +1,60 @@
+/**
+ * Time Complexity: O(1) for all operations (push, pop, peek, isEmpty) as we only manipulate the head pointer.
+ * Space Complexity: O(N) where N is the number of elements in the stack.
+ */
 public 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() 
     { 
-        //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. 
+        //Write code to push data to the stack.
+        StackNode newNode = new StackNode(data);
+        if(root==null){
+            root = newNode;
+        }else{
+            newNode.next = root;
+            root = newNode;
+        } 
     } 
 
     public int pop() 
     { 	
-	//If Stack Empty Return 0 and print "Stack Underflow"
+	   //If Stack Empty Return 0 and print "Stack Underflow"
         //Write code to pop the topmost element of stack.
-	//Also return the popped element 
+	   //Also return the popped element 
+      if(root==null){
+           System.out.print("Stack Underflow");
+          return 0;
+      }
+      int poppedValue = root.data;
+      root = root.next;  
+      return poppedValue;
     } 
 
     public int peek() 
     { 
         //Write code to just return the topmost element without removing it.
+        if(root!=null)
+          return root.data;
+        else
+          return -1;
     } 
 
 	//Driver code

Exercise_3.java

@@ -1,3 +1,10 @@
+/**
+ * Insert: Time Complexity: O(n) because we must traverse the entire list to find the end.
+ * Space Complexity: O(1) 
+ * 
+ * Print:Time Complexity: O(n) where n is the number of nodes.
+ * Space Complexity: O(1).
+ */
 import java.io.*; 
 
 // Java program to implement 
@@ -17,7 +24,8 @@ static class Node {
         // Constructor 
         Node(int d) 
         { 
-            //Write your code here 
+            this.data = d;
+            this.next = null;
         } 
     } 
 
@@ -34,7 +42,17 @@ public static LinkedList insert(LinkedList list, int data)
 
             // Insert the new_node at last node 
         // Return the list by head 
-
+        Node node = new Node(data);
+        if(list.head==null){
+            list.head = node;
+        }else{
+            Node last = list.head;
+            while(last.next!=null){
+                last = last.next;
+            }
+            last.next = node;
+        }
+       return list;
     } 
 
     // Method to print the LinkedList. 
@@ -45,6 +63,14 @@ public static void printList(LinkedList list)
             // Print the data at current node 
 
             // Go to next node 
+       Node currNode = list.head;     
+       while(currNode!=null){
+          Node curr = list.head;
+
+              System.out.println(head.data);
+              head = head.next;
+
+       }
     } 
 
     // Driver code