"Pre-course 1 done"

Exercise_1.java

@@ -1,42 +1,64 @@
+// Time Complexity: 
+//   push: O(1)
+//   pop: O(1)
+//   peek: O(1)
+// Space Complexity: O(n)
+// Did this code successfully run on Leetcode :No
+// Any problem you faced while coding this : No
+
+
+// Your code here along with comments explaining your approach
+// We implement a stack using a fixed-size array of MAX size.
+// 'top' keeps track of the index of the last inserted element.
+// When the stack is empty, top = -1.
+
 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 
-    } 
+    int a[] = new int[MAX]; // stack array
 
-    Stack() 
-    { 
-        //Initialize your constructor 
+    // Constructor
+    Stack() { 
+        top = -1;  // stack is empty
     } 
 
-    boolean push(int x) 
-    { 
-        //Check for stack Overflow
-        //Write your code here
+    // Check if stack is empty
+    boolean isEmpty() { 
+        return (top < 0);
+    } 
+
+    // Push element onto stack
+    boolean push(int x) { 
+        if (top >= MAX - 1) {   // stack overflow check
+            System.out.println("Stack Overflow");
+            return false;
+        }
+        a[++top] = x;  // increment top and add element
+        return true;
     } 
 
-    int pop() 
-    { 
-        //If empty return 0 and print " Stack Underflow"
-        //Write your code here
+    // Pop element from stack
+    int pop() { 
+        if (top < 0) {  // underflow check
+            System.out.println("Stack Underflow");
+            return 0;
+        }
+        return a[top--]; // return top element and decrease top
     } 
 
-    int peek() 
-    { 
-        //Write your code here
+    // Return top element without removing
+    int peek() { 
+        if (top < 0) {
+            System.out.println("Stack Underflow");
+            return 0;
+        }
+        return a[top];
     } 
 } 
 
 // Driver code 
 class Main { 
-    public static void main(String args[]) 
-    { 
+    public static void main(String args[]) { 
         Stack s = new Stack(); 
         s.push(10); 
         s.push(20); 

Exercise_1.py

@@ -1,20 +1,47 @@
+# Time Complexity: 
+#   push: O(1)
+#   pop: O(1)
+#   peek: O(1)
+# Space Complexity: O(n)
+# Did this code successfully run on Leetcode :No
+# Any problem you faced while coding this : No
+
+
+# Your code here along with comments explaining your approach
+# We implement a stack using a fixed-size array of MAX size.
+# 'top' keeps track of the index of the last inserted element.
+# When the stack is empty, top = -1.
 class myStack:
   #Please read sample.java file before starting.
   #Kindly include Time and Space complexity at top of each file
      def __init__(self):
-
+       self.stack = []
+       self.stackSize = 0
+
      def isEmpty(self):
+       return self.stackSize==0
 
      def push(self, item):
+       self.stack.append(item)
+       self.stackSize += 1
 
      def pop(self):
+        if self.isEmpty():
+          return None
+        self.stackSize -= 1
+        return self.stack.pop()
 
 
      def peek(self):
+       if self.isEmpty():
+         return None
+       return self.stack[-1]
 
      def size(self):
+       return self.stackSize
 
      def show(self):
+       return self.stack
 
 
 s = myStack()

Exercise_2.java

@@ -1,43 +1,68 @@
+// Time Complexity : Push - O(1), Pop - O(1)
+// Space Complexity : O(n)
+// Did this code successfully run on Leetcode : yes
+// Any problem you faced while coding this : no
+
+// Your code here along with comments explaining your approach
+// 1. Use a linked list where 'top' always points to the most recently pushed node.
+// 2. For push: create a new node, point its next to current top, and update top.
+// 3. For pop: return the data from top and move top to top.next.
+
 public class StackAsLinkedList { 
 
     StackNode root; 
 
     static class StackNode { 
         int data; 
-        StackNode next; 
+        StackNode next;
 
-        StackNode(int data) 
-        { 
-            //Constructor here 
+        StackNode(int data) { 
+            this.data = data;
+            this.next = null; 
         } 
     } 
 
-
-    public boolean isEmpty() 
-    { 
-        //Write your code here for the condition if stack is empty. 
+    // Check if stack is empty
+    public boolean isEmpty() { 
+        return root == null;
     } 
 
-    public void push(int data) 
-    { 
-        //Write code to push data to the stack. 
+    // Push element onto the stack
+    public void push(int data) { 
+        // Always insert at the beginning
+        StackNode new_node = new StackNode(data);
+
+        // New node should now become the root (top of stack)
+        new_node.next = root;
+        root = new_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 
+    // Pop top element
+    public int pop() { 
+        // If empty
+        if (isEmpty()) {
+            System.out.println("Stack Underflow");
+            return 0;
+        }
+
+        // Store current root and move root pointer
+        int popped = root.data;
+        root = root.next;
+
+        return popped;
     } 
 
-    public int peek() 
-    { 
-        //Write code to just return the topmost element without removing it.
+    // Peek top element
+    public int peek() { 
+        if (isEmpty()) {
+            System.out.println("Stack Underflow");
+            return 0;
+        }
+        return root.data;
     } 
 
-	//Driver code
-    public static void main(String[] args) 
-    { 
+    // Driver code
+    public static void main(String[] args) { 
 
         StackAsLinkedList sll = new StackAsLinkedList(); 
 
@@ -49,4 +74,4 @@ public static void main(String[] args)
 
         System.out.println("Top element is " + sll.peek()); 
     } 
-} 
+}

Exercise_2.py

@@ -1,32 +1,58 @@
+# Time Complexity : Push - O(1), Pop - O(1)
+# Space Complexity : O(n)
+# Did this code successfully run on Leetcode : yes
+# Any problem you faced while coding this : no
+
+# Your code here along with comments explaining your approach
+# 1. Use a linked list where 'top' always points to the most recently pushed node.
+# 2. For push: create a new node, point its next to current top, and update top.
+# 3. For pop: return the data from top and move top to top.next.
 
 class Node:
     def __init__(self, data):
-       self.data = data
-       self.next = None
+        self.data = data
+        self.next = None
 
 class Stack:
     def __init__(self):
+        self.top = None
 
     def push(self, data):
+        # Create new node and add it at the top of stack
+        new_node = Node(data)
+        new_node.next = self.top
+        self.top = new_node
 
     def pop(self):
+        # If the stack is empty, return None
+        if not self.top:
+            return None
+
+        popped_value = self.top.data
+        self.top = self.top.next   # Move top to next node
+        return popped_value
 
+
+# ------------------- Example Interaction Code (as you provided) -------------------
+
 a_stack = Stack()
 while True:
-    #Give input as string if getting an EOF error. Give input like "push 10" or "pop"
     print('push <value>')
     print('pop')
     print('quit')
     do = input('What would you like to do? ').split()
-    #Give input as string if getting an EOF error. Give input like "push 10" or "pop"
+
     operation = do[0].strip().lower()
+
     if operation == 'push':
         a_stack.push(int(do[1]))
+
     elif operation == 'pop':
         popped = a_stack.pop()
         if popped is None:
             print('Stack is empty.')
         else:
             print('Popped value: ', int(popped))
+
     elif operation == 'quit':
         break

Exercise_3.java

@@ -1,3 +1,13 @@
+// Time Complexity : O(1)- append, others - O(n)
+// Space Complexity :O(n)
+// Did this code successfully run on Leetcode :yes
+// Any problem you faced while coding this :yes
+
+
+// Your code here along with comments explaining your approach
+// The code creates a singly linked list where each node stores a value and a pointer to the next node.
+// Insertion takes O(n) because it traverses to the last node before appending, and printing also takes O(n) as it visits all nodes.
+// Space complexity is O(n) since each inserted element allocates a new node in memory.
 import java.io.*; 
 
 // Java program to implement 
@@ -17,34 +27,56 @@ static class Node {
         // Constructor 
         Node(int d) 
         { 
-            //Write your code here 
+            //Write your code here
+            this.data = d;
+            this.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);
+        new_node.next = null;
+
         // If the Linked List is empty, 
         // then make the new node as head 
-
+        if (list.head == null) {
+            list.head = new_node;
+        } 
+        else {
             // Else traverse till the last node 
             // and insert the new_node there 
-
+            Node last = list.head;
+            while (last.next != null) {
+                last = last.next;
+            }
+
             // Insert the new_node at last node 
+            last.next = new_node;
+        }
+
         // Return the list by head 
-
+        return list;
     } 
 
     // Method to print the LinkedList. 
     public static void printList(LinkedList list) 
     {  
         // Traverse through the LinkedList 
-
-            // Print the data at current node 
-
-            // Go to next node 
+        Node currNode = list.head;
+
+        System.out.print("LinkedList: ");
+
+        // Print the data at current node 
+        // then move to next node
+        while (currNode != null) {
+            System.out.print(currNode.data + " ");
+            currNode = currNode.next;
+        }
+
+        System.out.println();
     } 
 
     // Driver code 
@@ -53,11 +85,7 @@ public static void main(String[] args)
         /* Start with the empty list. */
         LinkedList list = new LinkedList(); 
 
-        // 
         // ******INSERTION****** 
-        // 
-
-        // Insert the values 
         list = insert(list, 1); 
         list = insert(list, 2); 
         list = insert(list, 3); 
@@ -67,4 +95,4 @@ public static void main(String[] args)
         // Print the LinkedList 
         printList(list); 
     } 
-}
+}