[C++知识库]232. Implement Queue using Stacks

使用二个栈来完成队列的功能，将栈1中元素全部取出放到栈2中，然后将新加的元素放到栈1中，再将栈2中所有元素依次放到栈1中。

class MyQueue {
public:
    MyQueue() {
        stack<int> s1;
        stack<int> s2;
    }
    
    void push(int x) {
         while(!s1.empty()) {
            s2.push(s1.top());
            s1.pop();
        }
        s2.push(x);
        while(!s2.empty()) {
            s1.push(s2.top());
            s2.pop();
        }
    }
    //取出队头 并返回值
    int pop() {
        int curr = s1.top();
        s1.pop();
        return curr;
    }
    //获得队头的值
    int peek() {
        return s1.top(); 
    }
    
    bool empty() {
        return st.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();
 */