Enumeration

LeetCode 2044 note: the maximum OR is the OR of everything; once backtracking reaches it, count the rest as 2^k

LeetCode 2094 note: enumerate the even numbers 100 to 998 instead and check that each digit needed is available; Counter subtraction keeps only positives

LeetCode 204 note: the sieve of Eratosthenes, crossing out multiples of each prime p starting at p², with the outer loop only up to √n