Leetcode #346: Moving Average From Data Stream

class MovingAverage {
    /** Initialize your data structure here. */
    private var stack = [Int]()
    private let size: Int
    private var sum = 0
    
    init(_ size: Int) {
        self.size = size
    }
    
    func next(_ val: Int) -> Double {
        if stack.count < self.size {
            stack.append(val)
            sum += val
            return Double(sum) / Double(stack.count)
        } else {
            sum -= stack.removeFirst()
            stack.append(val)
            sum += val
            return Double(sum) / Double(self.size)
        }
    }
}