双栈模拟队列

使用第一个栈存储push的元素，只有当需要pop时，才将第二个栈中的元素全部倒入到第二个栈中。n次操作总复杂度为O(n)

class MyQueue {
private:
    stack<int> st1,st2;

    int popSt2(){
        int ans=st2.top();
        st2.pop();
        return ans;
    }

    void transfer(){
        while(!st1.empty()){
            st2.push(st1.top());
            st1.pop();
        }
    }
public:
    /** Initialize your data structure here. */
    MyQueue() {

    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        st1.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        if(!st2.empty()){
            return popSt2();
        }
        transfer();
        return popSt2();
    }

    
    /** Get the front element. */
    int peek() {
        if(!st2.empty()){
            return st2.top();
        }
        transfer();
        return st2.top();
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return st1.empty() && st2.empty();
    }
};