Moving Average from Data Stream -- LeetCode

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

 class MovingAverage {
 private:
     queue<int> numsInWindow;
     int windowSize;
     double lastAverage;
 public:
     /** Initialize your data structure here. */
     MovingAverage(int size) {
         windowSize = size;
     }

     double next(int val) {
         if (numsInWindow.empty()) {
             numsInWindow.push(val);
             lastAverage = (double) val;
         }
         else if (numsInWindow.size() < windowSize) {
             double total = lastAverage * numsInWindow.size();
             numsInWindow.push(val);
             lastAverage = (total + val) / numsInWindow.size();
         }
         else {
             double total = lastAverage * numsInWindow.size();
             total -= numsInWindow.front();
             numsInWindow.pop();
             numsInWindow.push(val);
             lastAverage = (total + val) / numsInWindow.size();
         }
         return lastAverage;
     }
 };

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