225 队列实现栈

队列先进先出，栈先进后出。队列实现栈：插入元素的时候直接push进队列，同时记录最后一个入队的元素，此元素就是栈的top元素；队列为空的时候栈即空；比较麻烦的是pop，策略是依次取出队列最后一个元素（需要pop的元素）的元素，然后添加到队列后面，把原来队列最后面的元素放到第一个，然后pop掉；需要注意需要改变top元素的值，改成原来队列的倒数第二个元素

#include<iostream>
#include<queue>
using namespace std;

class MyStack {
public:
    /** Initialize your data structure here. */
    queue<int> que;
    int top_elem;
    MyStack() {

    }
    
    /** Push element x onto stack. */
    void push(int x) {
        que.push(x);
        top_elem = x;
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        int size = que.size();
        while(size > 2) {
            que.push(que.front());
            size --;
            que.pop();
        }
        if(size == 2) {
            top_elem = que.front();
            que.push(que.front());
            que.pop();
        }   
        int res = que.front();
        que.pop();
        return res;
    }
    
    /** Get the top element. */
    int top() {
        return top_elem;
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return que.empty();
    }
};

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack* obj = new MyStack();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->top();
 * bool param_4 = obj->empty();
 */

int main() {

}

232 用栈实现队列

s1用来push添加元素，s2用来top和pop，当s1 s2都为空的时候队列为空。

#include<iostream>
#include<stack>
using namespace std;

class MyQueue {
public:
    stack<int> s1, s2;
    /** Initialize your data structure here. */
    MyQueue() {

    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        s1.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        int res = peek();
        s2.pop();
        return res;
    }
    
    /** Get the front element. */
    int peek() {
        if(s2.empty()) {
            while (!s1.empty()) {
                s2.push(s1.top());
                s1.pop();
            }
        }
        return s2.top();
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return s1.empty() && s2.empty();
    }
};

/**
 * 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();
 */