leetcode 232. Implement Queue using Stacks

思路：两个栈,s1,s2，每次push的时候将其放入到s1，当pop或top的时候，将s1中的元素依次取出放入s2中，这样s2pop的顺序就是队列的顺序了。 

#include
class MyQueue {
public:
    stack s1,s2;
    int length =0;
    /** Initialize your data structure here. */
    MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        s1.push(x);
        length ++;
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        int res;
        if(s2.empty())
        {
            
            while(!s1.empty())
            {
                int tmp = s1.top();
                s2.push(tmp);
                s1.pop();
             }
            res = s2.top();
            s2.pop();
          }
        else
        {
            res = s2.top();
            s2.pop();
        }
        length--;
        return res;
    }
    
    /** Get the front element. */
    int peek() {
      int res;
        if(s2.empty())
        {
            
            while(!s1.empty())
            {
                int tmp = s1.top();
                s2.push(tmp);
                s1.pop();
             }
            res = s2.top();
        
          }
        else
        {
            res = s2.top();
        }
        return res;
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        if(length==0)return true;
        else return false;
    }
};

/**
 * 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();
 */