stacks

Author: shashankvishwakarma

src/main/java/dsa/stack/StackUsingArrayList.java

@@ -0,0 +1,46 @@
+package dsa.stack;
+
+import java.util.ArrayList;
+
+public class StackUsingArrayList {
+    public static void main(String[] args) {
+        Stack stack = new Stack();
+        stack.push(1);
+        stack.push(2);
+        stack.push(3);
+        stack.push(4);
+
+        while (!stack.isEmpty()) {
+            System.out.println(stack.peek());
+            stack.pop();
+        }
+    }
+
+    static class Stack {
+        ArrayList<Integer> list = new ArrayList<>();
+
+        public void push(int data) {
+            list.add(data);
+        }
+
+        public boolean isEmpty() {
+            return list.size() == 0;
+        }
+
+        public int pop() {
+            if (isEmpty()) {
+                return -1;
+            }
+            int top = list.remove(list.size() - 1);
+            return top;
+        }
+
+        public int peek() {
+            if (isEmpty()) {
+                return -1;
+            }
+            return list.get(list.size() - 1);
+        }
+    }
+
+}

src/main/java/dsa/stack/StackUsingLinkedList.java

@@ -0,0 +1,61 @@
+package dsa.stack;
+
+public class StackUsingLinkedList {
+    public static void main(String[] args) {
+        Stack.push(1);
+        Stack.push(2);
+        Stack.push(3);
+        Stack.push(4);
+
+        while (!Stack.isEmpty()) {
+            System.out.println(Stack.peek());
+            Stack.pop();
+        }
+    }
+
+    private static class Node {
+        int data;
+        Node next;
+
+        Node(int data) {
+            this.data = data;
+            next = null;
+        }
+    }
+
+    static class Stack {
+        public static Node head = null;
+
+        public static void push(int data) {
+            Node newNode = new Node(data);
+
+            if (head == null) {
+                head = newNode;
+                return;
+            }
+            newNode.next = head;
+            head = newNode;
+        }
+
+        public static boolean isEmpty() {
+            return head == null;
+        }
+
+        public static int pop() {
+            if (isEmpty()) {
+                return -1;
+            }
+            Node top = head;
+            head = head.next;
+            return top.data;
+        }
+
+        public static int peek() {
+            if (isEmpty()) {
+                return -1;
+            }
+            Node top = head;
+            return top.data;
+        }
+    }
+}