使用栈实现队列。

解答[Java]

import java.util.ArrayDeque;
import java.util.Deque;

class MyQueue {
    Deque<Integer> stack1;
    Deque<Integer> stack2;

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

    /** 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();
        } else {
            while (!stack1.isEmpty()) {
                stack2.push(stack1.pop());
            }
            return stack2.pop();
        }
    }

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

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

/**
 * 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.isEmpty();
 */