Design a data structure that stores a dynamically changing list of integers and can find the median in constant time, $O(1)$, as the list grows. To do that, implement a class named MedianOfStream with the following functionality:

• Constructor(): Initializes an instance of the class.

• insertNum(int num): Adds a new integer num to the data structure.

• findMedian(): Returns the median of all integers added so far.

Note: The median is the middle value in a sorted list of integers.

• For an odd-sized list (e.g., $[4, 5, 6]$), the median is the middle element: $5$.

• For an even-sized list (e.g., $[2, 4, 6, 8]$), the median is the average of the two middle elements: $(4 + 6) / 2 = 5$.

Constraints:

• $-10^5 \leq$ num $\leq 10^5$, where num is an integer received from the data stream.

• There will be at least one element in the data structure before the median is computed.

• At most, $500$ calls will be made to the function that calculates the median.