// Given an array of positive integers, where each value within the array represents a
// weight of the corresponding index, implement a structure which can be invoked multiple times
// to pick a random index within that array according to in proportion to its weight.
//
// array [1, 7, 2]
// In 10 times, 0 -- 1 time, 1 -- 7 times, 2 -- 2 times
A classic weighted random selection question: build a prefix-sum array, generate a random number within the weighted range, and use binary search to find the correct index. Commonly tested in Meta interviews for probability distribution and prefix-sum logic
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