Stack

题目描述[原题链接][https://www.acwing.com/problem/content/description/36/]

请用栈实现一个队列，支持如下四种操作：

• push(x) – 将元素x插到队尾；
• pop() – 将队首的元素弹出，并返回该元素；
• peek() – 返回队首元素；
• empty() – 返回队列是否为空；

注意：

• 你只能使用栈的标准操作：push to top，peek/pop from top, size 和 is empty；
• 如果你选择的编程语言没有栈的标准库，你可以使用list或者deque等模拟栈的操作；
• 输入数据保证合法，例如，在队列为空时，不会进行pop或者peek等操作；

样例

MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);
queue.peek();  // returns 1
queue.pop();   // returns 1
queue.empty(); // returns false

算法描述

初始化两个栈，压入操作直接压入第一个栈即可，弹出和取队列首部元素需要将数据转入另一个栈中，操作完后再将数据装回，判空即判断第一个栈是否为空即可，单向队列就可以实现了；

C++代码

class MyQueue {
public:
    stack<int> sta1;
    stack<int> sta2;
    /** Initialize your data structure here. */
    MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        sta1.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        int t;
        while(!sta1.empty()){
            t = sta1.top();
            sta1.pop();
            sta2.push(t);
        }
        t = sta2.top();
        sta2.pop();
        while(!sta2.empty()){
            sta1.push(sta2.top());
            sta2.pop();
        }
        return t;
    }
    
    /** Get the front element. */
    int peek() {
         int t;
        while(!sta1.empty()){
            t = sta1.top();
            sta1.pop();
            sta2.push(t);
        }
        t = sta2.top();
        while(!sta2.empty()){
            sta1.push(sta2.top());
            sta2.pop();
        }
        return t;
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        if(sta1.empty())return true;
        else false;
    }
};

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * bool param_4 = obj.empty();
 */

Java代码

class MyQueue {

    Stack<Integer> sta1;
    Stack<Integer> sta2;
    
    /** Initialize your data structure here. */
    public MyQueue() {
        sta1 = new Stack<>();
        sta2 = new Stack<>();
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        sta1.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        swap(sta1,sta2);
        int e = sta2.pop();
        swap(sta2,sta1);
        return e;
    }
    
    /** Get the front element. */
    public int peek() {
        swap(sta1,sta2);
        int e = sta2.peek();
        swap(sta2,sta1);
        return e;
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return sta1.empty();
    }
    
    public void swap(Stack<Integer> s1,Stack<Integer> s2){
        while(!s1.empty()){
            s2.push(s1.pop());
        }
    }
}

/**
 * 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();
 */