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Intelligent Substrings:

There are two types of characters in a particular language: special and normal. A character is special if its value is 1 and normal if its value is 0. Given string s, return the longest substring of s that contains at most k normal characters. Whether a character is normal is determined by a 26-digit bit string named charValue. Each digit in charValue corresponds to a lowercase letter in the English alphabet.

Example:

s = 'abcde'

alphabet = abcdefghijklmnopqrstuvwxyz

charValue = 10101111111111111111111111

For clarity, the alphabet is aligned with charValue below:

alphabet = abcdefghijklmnopqrstuvwxyz

charValue = 10101111111111111111111111

The only normal characters in the language (according to charValue) are b and d. The string s contains both of these characters. For k = 2, the longest substring of s that contains at most k = 2 normal characters is 5 characters long, abcde, so the return value is 5. If k = 1 instead, then the possible substrings are ['b', 'd', 'ab', 'bc', 'cd', 'de', 'abc', 'cde']. The longest substrings are 3 characters long, which would mean a return value of 3.