面试题 03.04：化栈为队

MyQueue queue = new MyQueue();
queue.push(1);
queue.push(2);
queue.peek();  // 返回 1
queue.pop();   // 返回 1
queue.empty(); // 返回 false

class MyQueue {
private:
    stack<int> st1;
    stack<int> st2;
public:
    /** Initialize your data structure here. */
    MyQueue() {
    }
    /** Push element x to the back of queue. */
    void push(int x) {
        while(!st1.empty()){
            st2.push(st1.top());
            st1.pop();
        }
        st1.push(x);
        while(!st2.empty()){
            st1.push(st2.top());
            st2.pop();
        }
    }
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        int res=st1.top();
        st1.pop();
        return res;
    }
    /** Get the front element. */
    int peek() {
        return st1.top();
    }
    /** Returns whether the queue is empty. */
    bool empty() {
        return st1.empty();
    }
};