思路：

一道很简单的题，就是栈是先进后出，队列是先进先出，用两个栈底相互对着，这样一个队列就产生了，右栈为空的情况，左栈栈底就是队首元素，所以我们需要将左栈全部压入右栈，右栈不为空，那么右栈就是之前压入过，这时右栈栈顶元素就是队首元素，大家画画图应该就明白了

class MyQueue {

    Stack<Integer> left;
    Stack<Integer> right;

    public MyQueue() {
        left = new Stack<>();
        right = new Stack<>();
    }

    public void push(int x) {
        left.push(x);
    }

    public int pop() {
        if (right.isEmpty()) {
            while (!left.isEmpty()) {
                right.push(left.pop());
            }

        }
        return right.pop();
    }

    public int peek() {
        if (right.isEmpty()) {
            while (!left.isEmpty()) {
                right.push(left.pop());
            }

        }
        return right.peek();
    }

    public boolean empty() {
        return left.isEmpty() && right.isEmpty();
    }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * boolean param_4 = obj.empty();
 */