Problem: 232. 用栈实现队列

题目描述

思路

利用两个栈，一个入栈一个出栈搭配着实现队列的相关操作：

1.创建两个栈stack1和stack2；
2.void push(int x):将要入队的元素先入栈stack1；
3.int pop()：当stack2为空时并且stack1不为空时将stack1中的元素均弹出并同时依次入栈stack2，最后将stack2的栈顶元素弹出
4.int peek()：同理pop；
5.boolean empty():只用两个栈均为空时才返回true

Code

class MyQueue {
    private Stack<Integer> stack1;
    private Stack<Integer> stack2;

    public MyQueue() {
        stack1 = new Stack<>();
        stack2 = new Stack<>();
    }

    /**
     * Push the element to the stack1
     *
     * @param x The element to be pushed
     */
    public void push(int x) {
        stack1.push(x);
    }

    /**
     * Pop the top element of stack2
     *
     * @return int
     */
    public int pop() {
        if (stack2.isEmpty()) {
            while (!stack1.isEmpty()) {
                stack2.push(stack1.pop());
            }
        }
        return stack2.pop();
    }

    /**
     * Peek the top element of stack2
     *
     * @return int
     */
    public int peek() {
        if (stack2.isEmpty()) {
            if (stack2.isEmpty()) {
                while (!stack1.isEmpty()) {
                    stack2.push(stack1.pop());
                }
            }
        }
        return stack2.peek();
    }

    /**
     * Check the empty of myQueue
     *
     * @return boolean
     */
    public boolean empty() {
        if (stack1.empty() && stack2.isEmpty()) {
            return true;
        }
        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.empty();
 */