function alternate(s: string): number {
    // Write your code here
    // get unique chars in input string
    const availableChars = Array.from(new Set<string>(s.split('')));
    
    // build available pair options
    const availablePairs: [string, string][] = [];
    availableChars.forEach((firstChar) => {
        availableChars.forEach((secondChar) => {
            if (firstChar === secondChar) {
                // same, not an option
                return;
            }
            // add pair combonation to list of available pairs
            availablePairs.push([firstChar, secondChar]);
        });
    });
    
    // build array of lenths for each available pair
    const pairsStrLenValues: number[] = availablePairs.map(([a, b]): number => {
        // regex to find char that are not the current pair
        const findUnavailableCharsRegex = new RegExp(`[^${a}${b}]`, 'gi');
        // regex to find duplicates of char a in string
        const findDuplicateACharRegex = new RegExp(`[${a}]{2}`);
        // regex to find duplicates of char b in string
        const findDuplicateBCharRegex = new RegExp(`[${b}]{2}`);
        
        // remove all char that are not in the current pair
        // @ts-ignore (due to lib version)
        const cleanedString = s.replaceAll(findUnavailableCharsRegex, ``);
        
        // check for duplicates of char a and char b to check if cleaned string is valid
        const doesCleanStringHaveDuplicateAChar = !!findDuplicateACharRegex.exec(cleanedString);
        const doesCleanStringHaveDuplicateBChar = !!findDuplicateBCharRegex.exec(cleanedString);
        
        const isCleanedStringValid: boolean = (
            !doesCleanStringHaveDuplicateAChar
            && !doesCleanStringHaveDuplicateBChar
        );
        if (!isCleanedStringValid) {
            // cleaned string not valid so return length of zero
            return 0;
        }
        // return length of valid string
        return cleanedString.length;
        
    });
    
    return Math.max(0, ...pairsStrLenValues);

}