记录窗口内的所有数字，同时为了加快计算，记录窗口内数字之和。
from collections import deque
class MovingAverage:
def __init__(self, size: int):
self.size = size
self.window = deque([])
self.ans = 0
def next(self, val: int) -> float:
self.window.append(val)
self.ans += val
while len(self.window) > self.size:
self.ans -= self.window.popleft()
return self.ans / len(self.window)