The textbook describes the main sections of discrete mathematics and describes the most important algorithms on discrete data structures. The basis of the book is the material of the lecture course, which the author has been reading at St. Petersburg State Technical University for the past decade and a half. The third edition has the same structure and sequence of presentation as the second. The book contains several dozen not very voluminous, but significant additions, clarifications and definitions. Updated exercises, bibliography and comments on it. For university students, practicing programmers and everyone who wants to study discrete mathematics.