346. Moving Average from Data Stream

class MovingAverage {
public:
/** Initialize your data structure here. */
MovingAverage(int size) {
q_size = size;
sum = 0;
}

double next(int val) {
double res;
if (q.size() < q_size) {  //当队列未达上限
q.push(val);
sum += val;  //利用上次求和结果
} else {  //当队列达上限
//减头
sum -= q.front();
q.pop();
//加尾
q.push(val);
sum += val;
}
res = (double)sum / q.size();
return res;
}
private:
queue<int> q;  //队列.模拟滑动窗口
int q_size;  //队列上限
int sum;  //暂存求和
};

/**
* Your MovingAverage object will be instantiated and called as such:
* MovingAverage obj = new MovingAverage(size);
* double param_1 = obj.next(val);
*/