yokolet's notelets

What is Catalan number ? The Catalan number belongs to the domain of combinatorial mathematics. It is a sequence of natural numbers such that: 1, 1, 2, 5, 14, 42, 132, 429, 1430, 4862, 16796, 58786, 208012, 742900, 2674440, 9694845, 35357670, 129644790, 477638700, 1767263190, ... The sequence appears in counting problems.

“Given an integer n, find whether n is a prime number or not” – This is a typical algorithm question. If the given number is small, repeating a division from 2 to n gives the answer.

Recently, I decided to give Clojure a try. This post is about testing HackerRank problems on a local env.

I used to write Clojure code for my day job, however, no Clojure at all for two years. Instead, almost all were done by Python.

A bunch of algorithm questions take a style of “maximum is a good thing.” Maximal sum, maximal length or maximal size are examples. This memo is about maximal size, precisely, square and rectangle.

Let’s revisit the Iterator pattern. “Iterator” is one of design patterns in object-oriented programming (OOP). Needless to say, extremely famous Gang of Four (Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides) are creators. As in the Wikipedia’s Iterator pattern,

Various types of string related problems exist. Among them, splitting it to make something valid would be a typical one. For example, a string of numbers is given, “make valid IP addresses” is the example. Very similar problem is “make valid lottery numbers.”

I’m going to write about two weird algorithm problems. Those seem quite weired at least to me, yet famous as algorithm questions. One is so-called egg dropping, another is finding a celebrity. I see these two problems here and there. From that, I guess those two are famous algorithm questions. But, at least, those two are quite weird. Some call them puzzle. (Yeah, maybe… I was totally puzzled.)

In my last post, Math Without Operator To Do It, I wrote about division an power implementations. There’s one more of this sort worth adding a memo here. It is calculating a square root without language provided operators.

Do division or power calculation without language provided operators – we see this sort of questions among algorithm problems. I wrote Addtion without +/- operators in the post about XOR related questions. “Divide without division” and “power without its operator or function” are examples as well.

Serialize, Deserialize a binary tree are a popular algorithm questions It depends on a programming language, but in most cases, a binary tree is expressed by an object tree. Each node can have at most two children: left node and right node. Once the binary tree is constructed, it is not language neutral anymore.