232. 用栈实现队列

https://leetcode-cn.com/problems/implement-queue-using-stacks/

class MyQueue {
    Stack<Integer> stack1;
    Stack<Integer> stack2;
    public MyQueue() {
        stack1 = new Stack<>();
        stack2 = new Stack<>();
    }
    public void push(int x) {
        // 1. S1 的全部元素进入 S2
        while (!stack1.isEmpty()){
            stack2.push(stack1.pop());
        }
        // 2. 将新元素压入 S1
        stack1.push(x);
        // 3. 再将 S2 中元素依次压入 S1
        while (!stack2.isEmpty()){
            stack1.push(stack2.pop());
        }
    }
    public int pop() {
        return stack1.pop();
    }
    public int peek() {
        return stack1.peek();
    }
    public boolean empty() {
        return stack1.isEmpty();
    }
}