TheAlgorithms · yanglbme · Jan 6, 2019 · Jan 4, 2019 · Jan 4, 2019 · Jan 4, 2019
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+/**
+ * This class implements a Stack using a regular array.
+ * <p>
+ * A stack is exactly what it sounds like. An element gets added to the top of
+ * the stack and only the element on the top may be removed. This is an example
+ * of an array implementation of a Stack. So an element can only be added/removed
+ * from the end of the array. In theory stack have no fixed size, but with an
+ * array implementation it does.
+ *
+ * @author Unknown
+ */
+public class StackArray {
+
+    /**
+     * Main method
+     *
+     * @param args Command line arguments
+     */
+    public static void main(String[] args) {
+        // Declare a stack of maximum size 4
+        StackArray myStackArray = new StackArray(4);
+
+        // Populate the stack
+        myStackArray.push(5);
+        myStackArray.push(8);
+        myStackArray.push(2);
+        myStackArray.push(9);
+
+        System.out.println("*********************Stack Array Implementation*********************");
+        System.out.println(myStackArray.isEmpty()); // will print false
+        System.out.println(myStackArray.isFull()); // will print true
+        System.out.println(myStackArray.peek()); // will print 9
+        System.out.println(myStackArray.pop()); // will print 9
+        System.out.println(myStackArray.peek()); // will print 2
+    }
+
+    /**
+     * The max size of the Stack
+     */
+    private int maxSize;
+
+    /**
+     * The array representation of the Stack
+     */
+    private int[] stackArray;
+
+    /**
+     * The top of the stack
+     */
+    private int top;
+
+    /**
+     * Constructor
+     *
+     * @param size Size of the Stack
+     */
+    public StackArray(int size) {
+        maxSize = size;
+        stackArray = new int[maxSize];
+        top = -1;
+    }
+
+    /**
+     * Adds an element to the top of the stack
+     *
+     * @param value The element added
+     */
+    public void push(int value) {
+        if (!isFull()) { // Checks for a full stack
+            top++;
+            stackArray[top] = value;
+        } else {
+            resize(maxSize * 2);
+            push(value); // don't forget push after resizing
+        }
+    }
+
+    /**
+     * Removes the top element of the stack and returns the value you've removed
+     *
+     * @return value popped off the Stack
+     */
+    public int pop() {
+        if (!isEmpty()) { // Checks for an empty stack
+            return stackArray[top--];
+        }
+
+        if (top < maxSize / 4) {
+            resize(maxSize / 2);
+            return pop();// don't forget pop after resizing
+        } else {
+            System.out.println("The stack is already empty");
+            return -1;
+        }
+    }
+
+    /**
+     * Returns the element at the top of the stack
+     *
+     * @return element at the top of the stack
+     */
+    public int peek() {
+        if (!isEmpty()) { // Checks for an empty stack
+            return stackArray[top];
+        } else {
+            System.out.println("The stack is empty, cant peek");
+            return -1;
+        }
+    }
+
+    private void resize(int newSize) {
+        // private int[] transferArray = new int[newSize]; we can't put modifiers here !
+        int[] transferArray = new int[newSize];
+
+        // for(int i = 0; i < stackArray.length(); i++){ the length isn't a method .
+        for (int i = 0; i < stackArray.length; i++) {
+            transferArray[i] = stackArray[i];
+            stackArray = transferArray;
+        }
+        maxSize = newSize;
+    }
+
+    /**
+     * Returns true if the stack is empty
+     *
+     * @return true if the stack is empty
+     */
+    public boolean isEmpty() {
+        return (top == -1);
+    }
+
+    /**
+     * Returns true if the stack is full
+     *
+     * @return true if the stack is full
+     */
+    public boolean isFull() {
+        return (top + 1 == maxSize);
+    }
+
+    /**
+     * Deletes everything in the Stack
+     * <p>
+     * Doesn't delete elements in the array
+     * but if you call push method after calling
+     * makeEmpty it will overwrite previous
+     * values
+     */
+    public void makeEmpty() { // Doesn't delete elements in the array but if you call
+        top = -1;             // push method after calling makeEmpty it will overwrite previous values
+    }
+}
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+import java.util.ArrayList;
+
+/**
+ * This class implements a Stack using an ArrayList.
+ * <p>
+ * A stack is exactly what it sounds like. An element gets added to the top of
+ * the stack and only the element on the top may be removed.
+ * <p>
+ * This is an ArrayList Implementation of a stack, where size is not
+ * a problem we can extend the stack as much as we want.
+ *
+ * @author Unknown
+ */
+public class StackArrayList {
+
+    /**
+     * Main method
+     *
+     * @param args Command line arguments
+     */
+    public static void main(String[] args) {
+
+        StackArrayList myStackArrayList = new StackArrayList();
+
+        myStackArrayList.push(5);
+        myStackArrayList.push(8);
+        myStackArrayList.push(2);
+        myStackArrayList.push(9);
+
+        System.out.println("*********************Stack List Implementation*********************");
+        System.out.println(myStackArrayList.isEmpty()); // will print false
+        System.out.println(myStackArrayList.peek()); // will print 9
+        System.out.println(myStackArrayList.pop()); // will print 9
+        System.out.println(myStackArrayList.peek()); // will print 2
+        System.out.println(myStackArrayList.pop()); // will print 2
+    }
+
+    /**
+     * ArrayList representation of the stack
+     */
+    private ArrayList<Integer> stackList;
+
+    /**
+     * Constructor
+     */
+    public StackArrayList() {
+        stackList = new ArrayList<>();
+    }
+
+    /**
+     * Adds value to the end of list which
+     * is the top for stack
+     *
+     * @param value value to be added
+     */
+    public void push(int value) {
+        stackList.add(value);
+    }
+
+    /**
+     * Pops last element of list which is indeed
+     * the top for Stack
+     *
+     * @return Element popped
+     */
+    public int pop() {
+
+        if (!isEmpty()) { // checks for an empty Stack
+            int popValue = stackList.get(stackList.size() - 1);
+            stackList.remove(stackList.size() - 1);  // removes the poped element from the list
+            return popValue;
+        }
+
+        System.out.print("The stack is already empty!");
+        return -1;
+    }
+
+    /**
+     * Checks for empty Stack
+     *
+     * @return true if stack is empty
+     */
+    public boolean isEmpty() {
+        return stackList.isEmpty();
+    }
+
+    /**
+     * Top element of stack
+     *
+     * @return top element of stack
+     */
+    public int peek() {
+        return stackList.get(stackList.size() - 1);
+    }
+}