Completed PreCourse-1

.gitignore

@@ -0,0 +1,13 @@
+
+# Ignore compiled Java files
+*.class
+
+# Ignore C++ executables
+a.out
+*.out
+*.exe
+Exercise_2
+
+# Ignore temp files
+*.o
+

Exercise_1.java

@@ -1,46 +1,69 @@
-class Stack { 
+
+// Time Complexity:
+// push() -> O(1)
+// pop()  -> O(1)
+// peek() -> O(1)
+// isEmpty() -> O(1)
+//
+// Space Complexity: O(MAX)
+
+import java.util.*;
+
+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 
-    } 
-
-    Stack() 
-    { 
-        //Initialize your constructor 
-    } 
-
-    boolean push(int x) 
-    { 
-        //Check for stack Overflow
-        //Write your code here
-    } 
-
-    int pop() 
-    { 
-        //If empty return 0 and print " Stack Underflow"
-        //Write your code here
-    } 
-
-    int peek() 
-    { 
-        //Write your code here
-    } 
-} 
-
-// Driver code 
-class Main { 
-    public static void main(String args[]) 
-    { 
-        Stack s = new Stack(); 
-        s.push(10); 
-        s.push(20); 
-        s.push(30); 
-        System.out.println(s.pop() + " Popped from stack"); 
-    } 
+    //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
+
+    Stack() {               // Constructor here
+        top = -1;
+    }
+
+    boolean push(int x) {
+        //Your code here
+        //Check Stack overflow as well
+        if (top >= MAX - 1) {
+            System.out.println("Stack Overflow");
+            return false;
+        }
+        a[++top] = x;
+        return true;
+    }
+
+    int pop() {
+        //Your code here
+        //Check Stack Underflow as well
+        if (top < 0) {
+            System.out.println("Stack Underflow");
+            return -1;
+        }
+        return a[top--];
+    }
+
+    int peek() {
+        //Your code here
+        //Check empty condition too
+        if (top < 0) {
+            System.out.println("Stack is Empty");
+            return -1;
+        }
+        return a[top];
+    }
+
+    boolean isEmpty() {
+        //Your code here
+        return (top < 0);
+    }
 }
+
+public class Exercise_1 {
+    public static void main(String[] args) {
+        Stack s = new Stack();
+        s.push(10);
+        s.push(20);
+        s.push(30);
+        System.out.println(s.pop() + " Popped from stack");
+    }
+}
+

Exercise_2.cpp

@@ -1,3 +1,18 @@
+// Time Complexity:
+// push()  -> O(1)
+// pop()   -> O(1)
+// peek()  -> O(1)
+// isEmpty() -> O(1)
+//
+// Space Complexity:
+// O(n) — where n is the number of elements pushed onto the stack,
+// because each push creates a new node in the linked list.
+//
+// Did this code successfully run on Leetcode?
+// This was not a LeetCode problem, but the code runs successfully on terminal.
+//
+// Any problems faced while coding this:
+// Understanding pointer manipulation and using StackNode** for push/pop.
 #include <bits/stdc++.h> 
 using namespace std; 
 
@@ -18,23 +33,38 @@ StackNode* newNode(int data)
 
 int isEmpty(StackNode* root) 
 { 
-    //Your code here 
+return !root;    //Your code here 
 } 
 
 void push(StackNode** root, int data) 
 { 
+	StackNode* stackNode = newNode(data);
+	stackNode->next = *root;
+	*root = stackNode;
     //Your code here 
 } 
 
 int pop(StackNode** root) 
 { 
-    //Your code here 
+    if (isEmpty(*root)){
+	    cout << "Stack Underflow\n";
+	    return INT_MIN;
+	    }
+    StackNode* temp = *root;
+    *root = (*root)->next;
+    int popped = temp -> data;
+    delete temp;
+    return popped;//Your code here 
 } 
 
 int peek(StackNode* root) 
 { 
-    //Your code here 
+    if (isEmpty(root)){
+	    cout << "Stack is empty\n";
+	    return INT_MIN;//Your code here 
 } 
+return root -> data;
+}
 
 int main() 
 { 
@@ -49,4 +79,4 @@ int main()
     cout << "Top element is " << peek(root) << endl; 
 
     return 0; 
-} 
+} 

Exercise_3.java

@@ -1,70 +1,73 @@
-import java.io.*; 
-
-// Java program to implement 
-// a Singly Linked List 
-public class LinkedList { 
-
-    Node head; // head of list 
-
-    // Linked list Node. 
-    // This inner class is made static 
-    // so that main() can access it 
-    static class Node { 
-
-        int data; 
-        Node next; 
-
-        // Constructor 
-        Node(int d) 
-        { 
-            //Write your code here 
-        } 
-    } 
-
-    // Method to insert a new node 
-    public static LinkedList insert(LinkedList list, int data) 
-    { 
-        // Create a new node with given 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 
+//Time Complexity:O(n)
+//Space: O(n)
+//On terminal it runs successfully
+//Problems Faced: Understanding pointers
+import java.io.*;
+
+// Java program to implement
+// a Singly Linked List
+public class Exercise_3 {
+
+    Node head; // head of list
+
+    // Linked list Node.
+    static class Node {
+        int data;
+        Node next;
+
+        // Constructor
+        Node(int d) {
+            this.data = d;
+            this.next = null;
+        }
+    }
+
+    // Method to insert a new node
+    public static Exercise_3 insert(Exercise_3 list, int data)
+    {
+        // Create a new node with given data
+        Node new_node = new Node(data);
+
+        // If list is empty
+        if (list.head == null) {
+            list.head = new_node;
+        }
+        else {
+            Node last = list.head;
+            while (last.next != null) {
+                last = last.next;
+            }
+            last.next = new_node;
+        }
+
+        return list;  // <-- FIXED
+    }
+
+    // Method to print the LinkedList.
+    public static void printList(Exercise_3 list)
+    {
+        Node current = list.head;
+
+        while (current != null) {
+            System.out.print(current.data + " ");
+            current = current.next;
+        }
+
+        System.out.println();
+    }
+
+    // Driver code
+    public static void main(String[] args)
+    {
+        Exercise_3 list = new Exercise_3();
+
+        list = insert(list, 1);
+        list = insert(list, 2);
+        list = insert(list, 3);
+        list = insert(list, 4);
+        list = insert(list, 5);
+
+        printList(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 
-
-            // Print the data at current node 
-
-            // Go to next node 
-    } 
-
-    // Driver code 
-    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); 
-        list = insert(list, 4); 
-        list = insert(list, 5); 
-
-        // Print the LinkedList 
-        printList(list); 
-    } 
-}