用两个栈实现队列

class MyQueue {
private:
    stack<int>in,out;

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

    }

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

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

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

    /** Returns whether the queue is empty. */
    bool empty() {
        return in.empty()&&out.empty();
    }
};

/**
 * 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();
 */

用队列实现栈

法一 两个队列

class MyStack {
private:
    queue<int>q1,q2;
public:
    MyStack() {

    }

    void push(int x) {
        q2.push(x);
        while(!q1.empty())
        {
            q2.push(q1.front());
            q1.pop();
        }
        swap(q1,q2);
    }

    int pop() {
        int res=q1.front();
        q1.pop();
        return res;
    }

    int top() {
        return q1.front();
    }

    bool empty() {
        return q1.empty();
    }
};

法二 一个队列

class MyStack {
private:
    queue<int>q;
public:
    MyStack() {

    }

    void push(int x) {
        int count=q.size();
        q.push(x);
        while(count--)
        {
            int t=q.front();
            q.pop();
            q.push(t);
        }
    }

    int pop() {
        int res=q.front();
        q.pop();
        return res;
    }

    int top() {
        return q.front();
    }

    bool empty() {
        return q.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();
 */