用两个栈实现队列

• 每次push，都push到stack1
• 每次pop，如果stack2为空，就把stack1中的所有元素“倒入”stack2，这样就使得最先进入stack1的元素又回到了栈顶

class MyQueue {
public:
    stack<int> stack1;
    stack<int> stack2;

    MyQueue() {
        // do intialization if necessary
        stack1 = stack<int>();
        stack2 = stack<int>();
    }

    void push(int element) {
        // write your code here
        stack1.push(element);
    }

    int pop() {
        // write your code here
        int pop_elem = top();
        stack2.pop();
        return pop_elem;
    }

    int top() {
        // write your code here
        if (stack2.empty()) {
            while (!stack1.empty()) {
                stack2.push(stack1.top());
                stack1.pop();
            }
        }
        return stack2.top();
    }
};