Strong Password Checker

For the classic checker, a password must have length 6 to 20, contain lowercase, uppercase, and digit characters, and avoid three identical consecutive characters. Count missing character classes and sum floor(runLength/3) over repeated runs as initial replacements. If length is below six, the answer is max(6-n, missing) because inserted characters can fill missing classes and break runs. If length is 6 through 20, return max(missing, replacements). If length exceeds 20, delete over = n-20 characters, prioritizing runs whose length modulo 3 is zero, then one, then two: one deletion reduces a replacement for remainder zero, two deletions for remainder one, and three thereafter. Subtract the replacement reductions, then return over + max(missing, remainingReplacements). The scan is O(n) time and O(1) space. Explain the deletion priority; it is the core greedy proof.