232 Implement Queue using Stacks 用栈来实现队列

使用栈来实现队列的如下操作：

push(x) -- 将一个元素放入队列的尾部。
pop() -- 从队列首部移除元素。
peek() -- 返回队列首部的元素。
empty() -- 返回队列是否为空。
注意:

你只能使用标准的栈操作-- 也就是只有push to top, peek/pop from top, size, 和 is empty 操作是可使用的。
你所使用的语言也许不支持栈。你可以使用 list 或者 deque (双端队列)来模拟一个栈，只要你仅使用栈的标准操作就可以。
假设所有操作都是有效的，比如 pop 或者 peek 操作不会作用于一个空队列上。

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

Java实现：

方法一：

class MyQueue {

    /** Initialize your data structure here. */
    private Stack<Integer> stk;
    public MyQueue() {
        stk=new Stack<Integer>();
    }

    /** Push element x to the back of queue. */
    public void push(int x) {
        stk.add(0,x);
    }

    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        return stk.pop();
    }

    /** Get the front element. */
    public int peek() {
        return stk.peek();
    }

    /** Returns whether the queue is empty. */
    public boolean empty() {
        return stk.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();
 */

方法二：

class MyQueue {

    /** Initialize your data structure here. */
    private Stack<Integer> stkPush;
    private Stack<Integer> stkPop;
    public MyQueue() {
        stkPush=new Stack<Integer>();
        stkPop=new Stack<Integer>();
    }

    /** Push element x to the back of queue. */
    public void push(int x) {
        stkPush.push(x);
    }

    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        if(stkPop.isEmpty()){
            while(!stkPush.isEmpty()){
                stkPop.push(stkPush.pop());
            }
        }
        return stkPop.pop();
    }

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

    /** Returns whether the queue is empty. */
    public boolean empty() {
        return stkPush.isEmpty()&&stkPop.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();
 */

C++实现：

class MyQueue {
public:
    /** Initialize your data structure here. */
    MyQueue() {

    }

    /** Push element x to the back of queue. */
    void push(int x) {
        stkPush.push(x);
    }

    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        if(stkPop.empty())
        {
            while(!stkPush.empty())
            {
                stkPop.push(stkPush.top());
                stkPush.pop();
            }
        }
        int val=stkPop.top();
        stkPop.pop();
        return val;
    }

    /** Get the front element. */
    int peek() {
        if(stkPop.empty())
        {
            while(!stkPush.empty())
            {
                stkPop.push(stkPush.top());
                stkPush.pop();
            }
        }
        return stkPop.top();
    }

    /** Returns whether the queue is empty. */
    bool empty() {
        return stkPush.empty()&&stkPop.empty();
    }
private:
    stack<int> stkPush;
    stack<int> stkPop;
};

/**
 * 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();
 * bool param_4 = obj.empty();
 */