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The brute force solution is to check every substring, which would TLE. How can we improve this solution?
In an equal count substring, the first character appears count times, the second character appears count times, and so on.
The length of an equal count substring is the number of unique characters multiplied by count.
The length of all equal count substrings are multiples of count.
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Problems involving sliding windows, substring counting, and frequency tracking are common in FAANG-style interviews. While the exact question may vary, the underlying techniques used in this problem are frequently tested.
A fixed-size frequency array of length 26 works best for lowercase English letters. It allows constant-time updates when the sliding window expands or shrinks, making the algorithm efficient and easy to implement.
The optimal approach uses a sliding window combined with character counting. By iterating over possible numbers of distinct characters and maintaining a fixed window size of k × count, we can efficiently check whether each character appears exactly the required number of times.
If a substring has k distinct characters and each must appear count times, the substring length must be k × count. Iterating over possible values of k helps restrict the window size and ensures we only check valid candidate substrings.