The book "Probabilistic Data Structures and Algorithms in Big Data Applications" is now available at Amazon and from local bookstores. More details at https://pdsa.gakhov.com In the presentation, I described popular and very simple data structures and algorithms to estimate the frequency of elements or find most occurred values in a data stream, such as Count-Min Sketch, Majority Algorithm, and Misra-Gries Algorithm. Each approach comes with some math that is behind it and simple examples to clarify the theory statements.