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题目：

题解：

class Solution {
private:mt19937 gen;uniform_int_distribution<int> dis;vector<int> pre;public:Solution(vector<int>& w): gen(random_device{}()), dis(1, accumulate(w.begin(), w.end(), 0)) {partial_sum(w.begin(), w.end(), back_inserter(pre));}int pickIndex() {int x = dis(gen);return lower_bound(pre.begin(), pre.end(), x) - pre.begin();}
};