This Hard-level string problem asks for the minimum operations (insertions, deletions, or replacements) to make the frequency of every character that appears in the string equal to some value f. You can choose any value for f, and you can also choose which characters to keep in the string. The goal is to reach a state where all remaining character types have the exact same frequency.
TikTok and Google use this question to evaluate a candidate's ability to handle complex optimization tasks involving multiple variables. It requires "Enumeration" (trying all possible frequencies) combined with "Dynamic Programming" or a very clever "Greer/Counting" strategy to minimize changes. It tests your ability to break down a high-level goal into smaller, manageable sub-problems.
The solution typically involves iterating through all possible target frequencies (from 1 up to the length of the string). For each target frequency f, you calculate the minimum operations needed to make a certain number of character types (say, k distinct characters) each have frequency f. This can be modeled as a DP problem where you decide for each character 'a' through 'z' whether to adjust its frequency to f, to 0 (delete all), or to something else.
String: "aabbccc", length 7.
f=2. Characters: {a, b, c}.
f=3.
A common error is only considering frequencies that already exist in the string. The optimal frequency f might be a value that doesn't match any current character count. Another mistake is failing to account for "replacements"—treating an insertion and a deletion as two separate operations when they could potentially be one replacement (depending on the specific rules of the problem variation). Handling all 26 lowercase letters efficiently in the DP is also a point of failure.
When an optimization problem has a limited range for one of its variables (like character frequency, which can't exceed string length), consider "Enumeration." Testing every possible value for that variable can often simplify the rest of the logic. Also, brush up on "Counting" problems and how to use frequency arrays (buckets) to organize string data.