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();*/
