Amortised time explained in simple terms: If you do an operation say a million times, you don’t really care about the worst-case or the best-case of that operation – what you care about is how much time is taken in total when you repeat the operation a million times. So it doesn’t matter if the … Read more
What is NP? NP is the set of all decision problems (questions with a yes-or-no answer) for which the ‘yes’-answers can be verified in polynomial time (O(nk) where n is the problem size, and k is a constant) by a deterministic Turing machine. Polynomial time is sometimes used as the definition of fast or quickly. … Read more
The basic idea is that you have two numbers to make up a primary key- a “high” number and a “low” number. A client can basically increment the “high” sequence, knowing that it can then safely generate keys from the entire range of the previous “high” value with the variety of “low” values. For instance, … Read more
Use itertools.groupby (see the itertools recipes): from itertools import groupby def all_equal(iterable): g = groupby(iterable) return next(g, True) and not next(g, False) or without groupby: def all_equal(iterator): iterator = iter(iterator) try: first = next(iterator) except StopIteration: return True return all(first == x for x in iterator) There are a number of alternative one-liners you might … Read more
If a number n is not a prime, it can be factored into two factors a and b: n = a * b Now a and b can’t be both greater than the square root of n, since then the product a * b would be greater than sqrt(n) * sqrt(n) = n. So in … Read more
public static string Reverse( string s ) { char[] charArray = s.ToCharArray(); Array.Reverse(charArray); return new string(charArray); }
According to Joshua Bloch’s Effective Java (a book that can’t be recommended enough, and which I bought thanks to continual mentions on stackoverflow): The value 31 was chosen because it is an odd prime. If it were even and the multiplication overflowed, information would be lost, as multiplication by 2 is equivalent to shifting. The … Read more
Simple check to see if two time periods overlap: bool overlap = a.start < b.end && b.start < a.end; or in your code: bool overlap = tStartA < tEndB && tStartB < tEndA; (Use <= instead of < if you change your mind about wanting to say that two periods that just touch each other … Read more
Now that MySQL 8.0 supports recursive queries, we can say that all popular SQL databases support recursive queries in standard syntax. WITH RECURSIVE MyTree AS ( SELECT * FROM MyTable WHERE ParentId IS NULL UNION ALL SELECT m.* FROM MyTABLE AS m JOIN MyTree AS t ON m.ParentId = t.Id ) SELECT * FROM MyTree; … Read more
Speaking as someone who spent 18 months working at a mapping company, which included working on the routing algorithm… yes, Dijkstra’s does work, with a couple of modifications: Instead of doing Dijkstra’s once from source to dest, you start at each end, and expand both sides until they meet in the middle. This eliminates roughly … Read more