// City Generator
// Given a list of city names and their corresponding populations, write a function to output a
// random city name subject to the following constraint: the probability of the function to output a
// city's name is based on its population divided by the sum of all cities' population.
// For example:
// NY: 7MM
// SF: 5MM
// LA: 8MM
// The probability to generate NY is 7/20, SF is 1/4.
This Microsoft interview problem focuses on weighted random selection. Given cities and their populations, the function should return a city name with probability proportional to its population. A common solution uses a prefix-sum array and binary search to map a random number to the correct weighted interval.
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