leetcode 用栈实现队列

class MyQueue {
private int front;

private Stack<Integer> in = null;
private Stack<Integer> out = null;
public MyQueue() {
    in = new Stack<>();
    out = new Stack<>();
}

public void push(int x) {
    in.push(x);
}
/**
 * Removes the element from in front of queue and returns that element.
 */
public int pop() {
    if (out.empty()) {
        int size = in.size();
        for (int i = 0; i < size; i++)
            out.push(in.pop());
    }
    return out.pop();
}
/**
 * Get the front element.
 */
public int peek() {
    if (out.empty()) {
        int size = in.size();
        for (int i = 0; i < size; i++)
            out.push(in.pop());
    }
    return out.peek();
}
/**
 * Returns whether the queue is empty.
 */
public boolean empty() {
    return in.empty() && out.empty();
}

}