用栈实现队列

import java.util.Stack;

/**
 * 用栈实现队列
 */
public class StackToQueue {

    public static class MyQueue<T> {

        public Stack<T> data;

        public Stack<T> help;

        public int size;

        public MyQueue() {
            data = new Stack<>();
            help = new Stack<>();
        }

        public void push(T value) {
            if (help.isEmpty()) {
                data.push(value);
            } else {
                help.push(value);
            }
            size++;
        }

        public T poll() {
            if (isEmpty()) {
                System.out.println("the queue is empty");
                return null;
            }
            size--;
            for (int i = 0; i < size; i++) {
                help.push(data.pop());
            }
            T value = data.pop();
            for (int i = 0; i < size; i++) {
                data.push(help.pop());
            }
            return value;
        }

        public boolean isEmpty() {
            return size == 0;
        }

    }

}

/* 如有错误，欢迎批评指正 */