力扣232. 用栈实现队列

使用栈实现队列的下列操作：

push(x) -- 将一个元素放入队列的尾部。
pop() -- 从队列首部移除元素。
peek() -- 返回队列首部的元素。
empty() -- 返回队列是否为空。

class MyQueue {

    Stack<Integer> stack1;
    Stack<Integer> stack2;
    /** Initialize your data structure here. */
    public MyQueue() {
        stack1 = new Stack<>();
        stack2 = new Stack<>();
    }

    /** Push element x to the back of queue. */
    public void push(int x) {
        stack1.push(x);
    }

    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        if(!stack2.isEmpty()){
            return stack2.pop();
        }
        while(!stack1.isEmpty()){
            stack2.push(stack1.pop());
        }
        return stack2.pop();
    }

    /** Get the front element. */
    public int peek() {
        if(!stack2.isEmpty()){
            return stack2.peek();
        }
        while(!stack1.isEmpty()){
            stack2.push(stack1.pop());
        }
        return stack2.peek();
    }

    /** Returns whether the queue is empty. */
    public boolean empty() {
        return stack1.isEmpty()&&stack2.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();
 */