PreCourse-1 Completed

Exercise_1.java

@@ -1,46 +1,82 @@
-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"); 
-    } 
+
+// Time Complexity : isEmpty - O(1), push - O(1), pop - O(1), peek - O(1)
+// Space Complexity :O(n)
+// Did this code successfully run on Leetcode :
+// Any problem you faced while coding this :not really, but i had to think for a bit on how i will implement it as this was the first time i implement the stack, once i could visualise the stack operations in my mind it was easy to code
+
+class Stack {
+    // Please read sample.java file before starting.
+    // Kindly include Time and Space complexity at top of each file
+    static final int MAX = 5;
+    int top;
+    int a[] = new int[MAX]; // Maximum size of Stack
+
+    boolean isEmpty() {
+        // simplified the logic
+        return top == -1;
+    }
+
+    Stack() {
+        this.top = -1;
+    }
+
+    boolean push(int x) {
+        boolean pushed;
+        if (top == (MAX - 1)) {
+            System.out.println("Stack overflow");
+            pushed = false;
+        } else {
+            top++;
+            a[top] = x;
+            pushed = true;
+        }
+
+        return pushed;
+    }
+
+    int pop() {
+        int value;
+        if (isEmpty()) {
+            System.out.println("Stack underflow");
+            return Integer.MIN_VALUE;
+        } else {
+            value = a[top];
+            top--;
+        }
+        return value;
+    }
+
+    int peek() {
+        int value;
+        if (isEmpty()) {
+            System.out.println("Stack underflow");
+            return Integer.MIN_VALUE;
+        } else {
+
+            value = a[top];
+        }
+        return value;
+    }
+}
+
+// Driver code
+class Exercise_1 {
+    public static void main(String args[]) {
+        Stack s = new Stack();
+        System.out.println(s.peek());
+        s.push(10);
+        System.out.println(s.peek());
+        s.push(20);
+        System.out.println(s.peek());
+        s.push(30);
+        System.out.println(s.peek());
+        s.push(40);
+        System.out.println(s.peek());
+        s.push(50);
+        System.out.println(s.pop() + " Popped from stack");
+        System.out.println(s.peek());
+        s.push(60);
+        System.out.println(s.peek());
+
+    }
 }

Exercise_2.java

@@ -1,52 +1,79 @@
-public class StackAsLinkedList { 
-
-    StackNode root; 
-
-    static class StackNode { 
-        int data; 
-        StackNode next; 
-
-        StackNode(int data) 
-        { 
-            //Constructor here 
-        } 
-    } 
-
-
-    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. 
-    } 
-
-    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 
-    } 
-
-    public int peek() 
-    { 
-        //Write code to just return the topmost element without removing it.
-    } 
-
-	//Driver code
-    public static void main(String[] args) 
-    { 
-
-        StackAsLinkedList sll = new StackAsLinkedList(); 
-
-        sll.push(10); 
-        sll.push(20); 
-        sll.push(30); 
-
-        System.out.println(sll.pop() + " popped from stack"); 
-
-        System.out.println("Top element is " + sll.peek()); 
-    } 
-} 
+
+// Time Complexity : isEmpty - O(1), push - O(1), pop - O(1), peek - O(1)
+// Space Complexity :O(n)
+// Did this code successfully run on Leetcode :
+// Any problem you faced while coding this :I have to put a more effort for this one because i am not i a habit of working with linked lists to first i have to figure out the linked list part firt, after that it was a bit easy as i had already implement stack for the previous exericise.
+public class StackAsLinkedList {
+
+    StackNode root;
+
+    static class StackNode {
+        int data;
+        StackNode next;
+
+        StackNode(int data) {
+            this.data = data;
+            next = null;
+
+        }
+    }
+
+    public boolean isEmpty() {
+        return root == null;
+        // Write your code here for the condition if stack is empty.
+    }
+
+    public void push(int data) {
+        // Null check added
+        StackNode temp = new StackNode(data);
+        if (isEmpty()) {
+            root = temp;
+
+        } else {
+            temp.next = root;
+            root = temp;
+        }
+        // Write code to push data to the stack.
+    }
+
+    public int pop() {
+        if (isEmpty()) {
+            System.out.println("Stack Underflow");
+            return 0;
+        } else {
+            int num;
+            num = root.data;
+            root = root.next;
+            return num;
+        }
+        // If Stack Empty Return 0 and print "Stack Underflow"
+        // Write code to pop the topmost element of stack.
+        // Also return the popped element
+    }
+
+    public int peek() {
+        // Write code to just return the topmost element without removing it.
+        if (isEmpty()) {
+            System.out.println("Stack Underflow");
+            return 0;
+        } else {
+            int num;
+            num = root.data;
+            return num;
+        }
+    }
+
+    // Driver code
+    public static void main(String[] args) {
+
+        StackAsLinkedList sll = new StackAsLinkedList();
+
+        sll.push(10);
+        sll.push(20);
+        sll.push(30);
+
+        System.out.println(sll.pop() + " popped from stack");
+
+        System.out.println("Top element is " + sll.peek());
+    }
+}