Count-distinct problem is a problem of finding the number of distinct elements in a data set or data stream, within which you might possibly see some repeated elements. For example, `[1, 3, 2, 1, 5, 2, 4]`

has 5 distinct elements `[1, 2, 3, 4, 5]`

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HyperLogLog is an algorithm that can solve Count Distinct problem. It can provide estimated count on a very large data stream.