ImplementQueueusingStacks.java

package problems.easy;

import java.util.Stack;

/**
 * Created by sherxon on 2016-12-29.
 */
public class ImplementQueueusingStacks {

 static class MyQueue {
 Stack<Integer> stack;
 Stack<Integer> helper;
 Integer peek = null;

 /**
 * Initialize your data structure here.
 */
 public MyQueue() {
 stack = new Stack<>();
 helper = new Stack<>();
 }

 /**
 * Push element x to the back of queue.
 */
 public void push(int x) {
 stack.push(x);
 if (peek == null) peek = x;
 }

 /**
 * Removes the element from in front of queue and returns that element.
 */
 public int pop() {
 helper.clear();

 while (!stack.isEmpty())
 helper.push(stack.pop());
 int x = helper.pop();

 if (!helper.isEmpty())
 peek = helper.peek();
 else peek = null;

 while (!helper.isEmpty())
 stack.push(helper.pop());
 return x;
 }

 /**
 * Get the front element.
 */
 public int peek() {
 return peek;
 }

 /**
 * Returns whether the queue is empty.
 */
 public boolean empty() {
 return stack.isEmpty();
 }
 }

 /**
 * This is the second optimized solution with
 * */
 public class MyQueue2 {
 Stack<Integer> stack;
 Stack<Integer> helper;

 /** Initialize your data structure here. */
 public MyQueue2() {
 stack= new Stack<>();
 helper= new Stack<>();
 }

 /** Push element x to the back of queue. */
 public void push(int x) {
 stack.push(x);
 }

 /** Removes the element from in front of queue and returns that element. */
 public int pop() {
 if(helper.isEmpty()){

 while(!stack.isEmpty())
 helper.push(stack.pop());
 }

 return helper.pop();
 }

 /** Get the front element. */
 public int peek() {
 if(helper.isEmpty()){

 while(!stack.isEmpty())
 helper.push(stack.pop());
 }
 return helper.peek();
 }

 /** Returns whether the queue is empty. */
 public boolean empty() {
 return helper.isEmpty() && stack.isEmpty();
 }
 }
}