Project Euler tactics

Some generic hints:

• Even if HashSet<int>/Dict<int,int> should in principle be fast the overhead is high enough that often it's much faster to use int[] or bool[]
  • For primes and checks rather than HashSet<int> with primes and IsPrime(n) => set.Contains(n) it's faster with bool[] and IsPrime(n) => primes[n]
  • For storing say divisors for some numbers it still might be faster to just use int[] over dictionary with Divisors(n) => divisors[n]>-1?divisors[n]:CalculateDivisors(n)
• Use memoization to short circuit expensive recursive calculations
• It's often cheaper to pre-generate all values than calculate individual.
  • Sieve primes rather than check individually
  • Find NumDivisors or SumOfDivisors across a range by iterating rather than for individual keys