#291 · #528 · Random Pick with Weight(按权重随机选择)

You are given a 0-indexed array of positive integers w where w[i] describes the weight of index i. Implement pickIndex() which returns an index chosen at random with probability proportional to its weight. Constraints: 1 <= w.length <= 10^4, 1 <= w[i] <= 10^5; at most 10^4 calls.