ImplementStackUsingQueues.java

package problems.easy;

import java.util.LinkedList;
import java.util.Queue;

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

 static class MyStack {
 Queue<Integer> q;
 Queue<Integer> temp;

 /**
 * Initialize your data structure here.
 */
 public MyStack() {
 q = new LinkedList<>();
 temp = new LinkedList<>();
 }

 /**
 * Push element x onto stack.
 */
 public void push(int x) {
 if (q.isEmpty()) q.add(x);
 else {
 while (!q.isEmpty()) // copy all to helper
 temp.add(q.poll());
 q.add(x); // add element
 while (!temp.isEmpty()) // copy back all to helper
 q.add(temp.poll());
 }

 }

 /**
 * Removes the element on top of the stack and returns that element.
 */
 public int pop() {
 return q.poll();
 }

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

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