用两个队列实现栈

 

class MyStack {
private:
    queue<int> q1, q2;
public:
    /** Initialize your data structure here. */
    MyStack() {

    }
    
    /** Push element x onto stack. */
    void push(int x) {
        q1.push(x);
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        int num = 0;
        while (!q1.empty()) {
            if (q1.front() == q1.back()) {
                num = q1.front();
                q1.pop();
                continue;
            }
            q2.push(q1.front());
            q1.pop();
        }
        while (!q2.empty()) {
            q1.push(q2.front());
            q2.pop();
        }
        return num;
    }
    
    /** Get the top element. */
    int top() {
        int num = 0;
        while (!q1.empty()) {
            if (q1.front() == q1.back()) {
                num = q1.front();
            }
            q2.push(q1.front());
            q1.pop();
        }
        while (!q2.empty()) {
            q1.push(q2.front());
            q2.pop();
        }
        return num;
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return q1.empty() ? true : false;
    }
};

/**
 * 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();
 */

单队列

class MyStack {
private:
    queue<int> q;
    
public:
    /** Initialize your data structure here. */
    MyStack() {
        
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        // 反转队列，将新元素插到队头
        
        int size = q.size();
        q.push(x);
        while (size--){
            int temp = q.front();
            q.pop();
            q.push(temp);
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        int popEle = q.front();
        q.pop();
        
        return popEle;
    }
    
    /** Get the top element. */
    int top() {
        return q.front();
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return q.empty();
    }
};