用队列实现栈，用栈实现队列

本文包含的Leetcode题目

1. 用队列实现栈（Leetcode 225）
2. 用栈实现队列（Leetcode 232）

Leetcode 225：用队列实现栈

准备两个queue：q1， q2
每次将元素先放入q2，然后将q1元素全部放入q2，再将q1和q2交换，这样能够保证每次最后进入的元素都在队列q1的最前端，实现栈逻辑。

class MyStack {
    
    
public:
    queue<int> queue1;
    queue<int> queue2;

    /** Initialize your data structure here. */
    MyStack() {
    
    

    }

    /** Push element x onto stack. */
    void push(int x) {
    
    
        queue2.push(x);
        while (!queue1.empty()) {
    
    
            queue2.push(queue1.front());
            queue1.pop();
        }
        swap(queue1, queue2);
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
    
    
        int r = queue1.front();
        queue1.pop();
        return r;
    }
    
    /** Get the top element. */
    int top() {
    
    
        int r = queue1.front();
        return r;
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
    
    
        return queue1.empty();
    }
};

Leetcode 232：用栈实现队列

准备两个栈s1,s2 s1用来存放数据，s2用来颠倒数据并弹出

class MyQueue {
    
    
public:
    stack<int> s1;
    stack<int> 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() {
    
    
        if (s2.empty()) {
    
    
            while (!s1.empty()) {
    
    
                s2.push(s1.top());
                s1.pop();
            }
        }
        int res = s2.top();
        s2.pop();
        return res;
    }
    
    /** Get the front element. */
    int peek() {
    
    
        int res = this->pop();    //peek操作可以直接调用pop() 
        s2.push(res); 
        return res;
    }
    
    /** 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();
 */