思路

1. 由于栈是先进后出，把栈中的元素出栈再压入另一栈中，就相当于把原栈中元素换了个顺序，原栈中先进的元素在另一栈中先出了，就符合队列的规则了。

C++代码

class MyQueue {
public:
    /** Initialize your data structure here. */
    stack<int> s1, s2;
    MyQueue() {

    }

    /** Push element x to the back of queue. */
    void push(int x) {
        s1.push(x);
    }

    //将s1中的元素弹出，压入s2中
    void copy(stack<int> &s1, stack<int> &s2)
    {
        while(!s1.empty())
        {
            s2.push(s1.top());
            s1.pop();
        }
    }
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        copy(s1, s2);
        int t = s2.top();
        s2.pop();
        copy(s2, s1);
        return t;
    }

    /** Get the front element. */
    int peek() {
        copy(s1, s2);
        int t = s2.top();
        copy(s2, s1);
        return t;
    }

    /** Returns whether the queue is empty. */
    bool empty() {
        return s1.empty();
    }
};

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * bool param_4 = obj.empty();
 */