queue_using_2_stacks

import java.util.NoSuchElementException;
import java.util.Stack;

public class Queue<T> {
    Stack<T> pushStack;
    Stack<T> pullStack;

    public Queue() {
        this.pushStack = new Stack<T>();
        this.pullStack = new Stack<T>();
    }

    private void flush() {
        while (!this.pushStack.empty()) {
            T next = this.pushStack.pop();
            this.pullStack.add(next);
        }
    }

    public void add(T elem) {
        this.pushStack.add(elem);
    }

    private T pop() {
        if (!this.pullStack.empty()) {
            return this.pullStack.pop();
        }
        this.flush();
        if (!this.pullStack.empty()) {
            return this.pullStack.pop();
        }
        return null;
    }

    private T head() {
        if (!this.pullStack.empty()) {
            return this.pullStack.peek();
        }
        this.flush();
        if (!this.pullStack.empty()) {
            return this.pullStack.peek();
        }
        return null;
    }

    public T poll() {
        return this.pop();
    }

    public T remove() {
        T elem = this.pop();
        if (elem != null) {
            return elem;
        }
        throw new NoSuchElementException();
    }

    public T peek() {
        return this.head();
    }

    public T element() {
        T elem = this.head();
        if (elem != null) {
            return elem;
        }
        throw new NoSuchElementException();
    }

}