LeetCode 225 Implement Stack using Queues 用队列实现栈

1、两个队列实现，始终保持一个队列为空即可

class MyStack {
public:
    /** Initialize your data structure here. */
    MyStack() {
        
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        if(que1.empty())
        {
            que1.push(x);
            while(!que2.empty())
            {
                int val=que2.front();
                que2.pop();
                que1.push(val);
            }
        }
        else
        {
            que2.push(x);
            while(!que1.empty())
            {
                int val=que1.front();
                que1.pop();
                que2.push(val);
            }
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        if(que1.empty())
        {
            int val=que2.front();
            que2.pop();
            return val;
        }
        if(que2.empty())
        {
            int val=que1.front();
            que1.pop();
            return val;
        }        
    }
    
    /** Get the top element. */
    int top() {
        if(que1.empty())
            return que2.front();
        if(que2.empty())
            return que1.front();
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return que1.empty()&&que2.empty();
    }
private:
    queue<int> que1;
    queue<int> que2;
};

/**
 * 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();
 */

2、一个队列实现栈

class MyStack {
public:
    /** Initialize your data structure here. */
    MyStack() {
        
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        que.push(x);
        for(int i=0;i<que.size()-1;++i)
        {
            que.push(que.front());
            que.pop();
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        int val=que.front();
        que.pop();
        return val;
    }
    
    /** Get the top element. */
    int top() {
        return que.front();
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return que.empty();
    }
private:
    queue<int> que;
};

/**
 * 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();
 */