Leetcode_#232_用棧實現隊列

原題：#232_用棧實現隊列

• 棧：先進後出
• 隊列：先進先出
• 使用兩個棧A，B。進棧時進入A，出棧時先從A出，進入B，再從B出。這時就可以實現先進先出了

class MyQueue {
    private Stack<Integer> in = new Stack<>();
    private Stack<Integer> out = new Stack<>();
    public void push(int x) {
        in.push(x);
    }
    public int pop() {
        in2out();
        return out.pop();
    }
    public int peek() {
        in2out();
        return out.peek();
    }
    private void in2out() {
        if (out.isEmpty()) {
            while (!in.isEmpty()) {
                out.push(in.pop());
            }
        }
    }
    public boolean empty() {
        return in.isEmpty() && out.isEmpty();
    }
}