TheAlgorithms · varunu28 · Oct 20, 2017 · Oct 19, 2017 · Oct 20, 2017
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+import java.util.Stack;
+
+/**
+ * This implements Queue using two Stacks.
+ *
+ * Big O Runtime:
+ *      insert(): O(1)
+ *      remove(): O(1) amortized
+ *      isEmpty(): O(1)
+ *
+ * A queue data structure functions the same as a real world queue.
+ * The elements that are added first are the first to be removed.
+ * New elements are added to the back/rear of the queue.
+ *
+ * @author sahilb2
+ *
+ */
+class QueueWithStack {
+
+    // Stack to keep track of elements inserted into the queue
+    private Stack inStack;
+    // Stack to keep track of elements to be removed next in queue
+    private Stack outStack;
+
+    /**
+	 * Constructor
+	 */
+    public QueueWithStack() {
+        this.inStack = new Stack();
+        this.outStack = new Stack();
+    }
+
+    /**
+     * Inserts an element at the rear of the queue
+     *
+     * @param x element to be added
+     */
+    public void insert(Object x) {
+        // Insert element into inStack
+        this.inStack.push(x);
+    }
+
+    /**
+     * Remove an element from the front of the queue
+     *
+     * @return the new front of the queue
+     */
+    public Object remove() {
+        if(this.outStack.isEmpty()) {
+            // Move all elements from inStack to outStack (preserving the order)
+            while(!this.inStack.isEmpty()) {
+                this.outStack.push( this.inStack.pop() );
+            }
+        }
+        return this.outStack.pop();
+    }
+
+    /**
+     * Returns true if the queue is empty
+     *
+     * @return true if the queue is empty
+     */
+    public boolean isEmpty() {
+        return (this.inStack.isEmpty() && this.outStack.isEmpty());
+    }
+
+}
+
+/**
+ * This class is the example for the Queue class
+ *
+ * @author sahilb2
+ *
+ */
+public class QueueUsingTwoStacks {
+
+    /**
+     * Main method
+     *
+     * @param args Command line arguments
+     */
+    public static void main(String args[]){
+        QueueWithStack myQueue = new QueueWithStack();
+        myQueue.insert(1);
+        // instack: [(top) 1]
+        // outStack: []
+        myQueue.insert(2);
+        // instack: [(top) 2, 1]
+        // outStack: []
+        myQueue.insert(3);
+        // instack: [(top) 3, 2, 1]
+        // outStack: []
+        myQueue.insert(4);
+        // instack: [(top) 4, 3, 2, 1]
+        // outStack: []
+
+        System.out.println(myQueue.isEmpty()); //Will print false
+
+        System.out.println(myQueue.remove()); //Will print 1
+        // instack: []
+        // outStack: [(top) 2, 3, 4]
+
+        myQueue.insert(5);
+        // instack: [(top) 5]
+        // outStack: [(top) 2, 3, 4]
+
+        myQueue.remove();
+        // instack: [(top) 5]
+        // outStack: [(top) 3, 4]
+        myQueue.remove();
+        // instack: [(top) 5]
+        // outStack: [(top) 4]
+        myQueue.remove();
+        // instack: [(top) 5]
+        // outStack: []
+        myQueue.remove();
+        // instack: []
+        // outStack: []
+
+        System.out.println(myQueue.isEmpty()); //Will print true
+
+    }
+}