This is a very comprehensive 94-part course on competitive programming. It gets you from knowing basic programming to being a yellow-red rated coder on Codeforces / CodeChef / TopCoder / etc.

The primary objectives of this course are to learn about 30 different algorithms and data structures. The algorithm tutori...

Defining Greedy Algorithm

An algorithm is called greedy if it follows the problem-solving heuristic of making the locally optimal choice at each stage with the aim of finding a global optimum.

In most situations, a greedy strategy does not lead to the optimal solution. Hence, it is extremely important to reason about the correctness of the greedy strategy before using it to solve a problem.

Problem 1: Too much to do!

Let's say you have to maximise the number of tasks that you complete in a 10-hour duration. You are given an array T = [2, 4, 1, 6] where T_i denotes the time required to complete the i^th task.

The answer to this question would be 3 as you would choose to do tasks T₂, T₀ and T₁ — which take 1, 2 and 4 hours respect...

The motivation for binary indexed trees is similar to that of segment trees. However, note that segment trees are much more flexible than binary indexed trees, and usually its true that any problem that can be solved with BIT can be solved with segment trees but not the other way around.

Video tutorial: This is a superb tutorial, giving the motivation, walking through example, and going step-by-step through the pseudocode.

While many ciphers have been created based upon the Feistel structure, the most famous of these is the Data Encryption Standard (DES). DES was based off of the original Lucifer cipher developed by Feistel and Coppersmith and submitted as an entry to the US National Bureau of Standards as a candidate for the US official encryption standard. After some modification (to improve security against differential cryptanalysis), it was selected and published as a standard in 1977.

Encryption with DES

The DES algorithm is a 16-round Feistel cipher. It takes as input a 64-bit input and a 64-bit secret key, and consists of three main stages:

• The initial permutation
• The round function (repeated 16 times)
• The final permutation

A diagram of how these stages fit together with the ...