numberFrequencyMap and frequencyCountMap.add is called:
a. Increment the frequency of the number in numberFrequencyMap.
b. Decrement the count of the old frequency in frequencyCountMap.
c. Increment the count of the new frequency in frequencyCountMap.deleteOne is called:
a. Decrement the frequency of the number in numberFrequencyMap if it's greater than 0.
b. Decrement the count of the old frequency in frequencyCountMap.
c. Increment the count of the new frequency in frequencyCountMap.hasFrequency is called, return true if the frequency exists in frequencyCountMap and its count is greater than 0.