stack with operations on middle element: done

Author: Ram swaroop

src/me/ramswaroop/linkedlists/StackWithOperationOnMiddleElement.java

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+package me.ramswaroop.linkedlists;
+
+import me.ramswaroop.common.DoubleLinkedNode;
+
+import java.util.EmptyStackException;
+
+import static java.lang.System.out;
+
+/**
+ * Created by IntelliJ IDEA.
+ *
+ * @author: ramswaroop
+ * @date: 7/24/15
+ * @time: 1:06 PM
+ * <p/>
+ * <p/>
+ * Implement a stack with below operations in O(1) time complexity:
+ * 1) push() which adds an element to the top of stack.
+ * 2) pop() which removes an element from top of stack.
+ * 3) findMiddle() which will return middle element of the stack.
+ * 4) deleteMiddle() which will delete the middle element.
+ * Push and pop are standard stack operations.
+ *
+ * The idea is to use a double linked list to represent a stack with pointer pointing to the middle node.
+ */
+public class StackWithOperationOnMiddleElement<E extends Comparable<E>> {
+
+    int i = -1;
+    DoubleLinkedNode<E> top, mid;
+
+    public void push(E item) {
+
+        top = new DoubleLinkedNode<>(null, item, top);
+        if (top.next != null) {
+            top.next.prev = top;
+        } else {
+            mid = top;
+        }
+
+        if (i % 2 == 0) {
+            mid = mid.prev;
+        }
+        i++;
+    }
+
+    public E pop() {
+        if (top == null) {
+            throw new EmptyStackException();
+        }
+
+        DoubleLinkedNode<E> topNode = top;
+        if (top.next != null) {
+            top.next.prev = null;
+        }
+        top = top.next;
+
+        i++;
+        if (i % 2 == 0) {
+            mid = mid.next;
+        }
+
+        return topNode.item;
+    }
+
+    public E getMiddleElement() {
+        if (mid == null) {
+            throw new EmptyStackException();
+        }
+        return mid.item;
+    }
+
+    /**
+     * Prints the content of the stack.
+     */
+    public void print() {
+        DoubleLinkedNode<E> curr = top;
+        out.print("[");
+        if (curr == null) {
+            out.println("]");
+            return;
+        }
+        // prints the list from first node
+        while (curr.next != null) {
+            out.print(curr.item.toString() + ",");
+            curr = curr.next;
+        }
+        out.println(curr.item.toString() + "]");
+        // prints the list from last node
+        out.print("[");
+        while (curr.prev != null) {
+            out.print(curr.item.toString() + ",");
+            curr = curr.prev;
+        }
+        out.println(curr.item.toString() + "]");
+    }
+
+    public static void main(String a[]) {
+        StackWithOperationOnMiddleElement<Integer> stack = new StackWithOperationOnMiddleElement<>();
+        stack.push(2);
+        stack.push(3);
+        stack.push(4);
+        stack.push(5);
+        stack.push(6);
+        stack.print();
+        System.out.println("Mid: " + stack.getMiddleElement());
+        stack.pop();
+        stack.print();
+        System.out.println("Mid: " + stack.getMiddleElement());
+        stack.push(7);
+        stack.print();
+        System.out.println("Mid: " + stack.getMiddleElement());
+        stack.pop();
+        stack.pop();
+        stack.pop();
+        stack.print();
+        System.out.println("Mid: " + stack.getMiddleElement());
+    }
+}