function longestPalindromicSubstring(input) {
    let longest = "";
    let lastChar = '';
    for (let i = 0; i < input.length; i++) {
        const char = input[i];
        const nextChar = input[i + 1]

        let substr = '';
        let j = 0;

        // e.g. "bananas" or "racecar"
        if (lastChar && nextChar && lastChar === nextChar) {
            substr = lastChar + char + nextChar;
            while (input[i - j] && input[i + j] && input[i - j] === input[i + j]) {
                substr = input.slice(i - j, i + j + 1)
                j++;
            }
        }
        // e.g. "millions"
        else if (char && lastChar && char === lastChar) {
            substr = lastChar + char;
            while (input[i - j - 1] && input[i + j] && input[i - j - 1] === input[i + j]) {
                substr = input.slice(i - j - 1, i + j + 1);
                j++;
            }
        }

        if (substr.length > longest.length)
            longest = substr;

        lastChar = char;
    }
    return longest;
}