Given a stream of integers and a window size, calculate the moving average of all integers in the sliding window.
For example,
MovingAverage m = new MovingAverage(3);
m.next(1) = 1
m.next(10) = (1 + 10) / 2
m.next(3) = (1 + 10 + 3) / 3
m.next(5) = (10 + 3 + 5) / 3

思路：

queue size大于window size，把queue顶部的元素弹出，同时从summary中减去这个元素，除以size大小得到average。

Complexity: time O(1), space O(window size)

class MovingAverage {
    private Queue<Integer> queue;
    private double sum = 0.0;
    private int windowSize = 0;
    
    
    /** Initialize your data structure here. */
    public MovingAverage(int size) {
        queue = new LinkedList<Integer>();
        this.windowSize = size;
    }
    
    public double next(int val) {
        queue.add(val);
        if (queue.size() > this.windowSize) {
            sum = sum - queue.poll();
        }
        sum = sum + val;
        return sum / queue.size();
    }
}

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