# Functions
NewAlias 新建一个抽样器，weightList是权重列表，src是随机数种子.
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RandInt 闭区间.
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RandString 生成长度为n的随机字符串.
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WeightedChoice 根据权重随机，返回对应选项的索引，O(n).
# Structs
AliasTable is a discrete distribution.