https://leetcode.com/problems/longest-palindromic-substring/

longest-palindromic-substring.js

/**
 * @param {string} s
 * @return {string}
 */
 var longestPalindrome = function(s) {
    const emptyArr = s.split('').fill(0)
    var [unevenMap, evenMap]  = [[...emptyArr], [...emptyArr]]
    
    const increaseUneven = (index, level) => {
        // hack cos NaN === NaN is false
        const prev = s[index - level] || NaN 
        const next = s[index + level] || NaN
        return prev === next
    }
    
    const increaseEven = (index, level) => {
        const prev = s[index - level + 1] || NaN
        const next = s[index + level] || NaN
        return prev === next
    }
    
    const findChampion = m => m
        .map((n, i) => ({n, i}))
        .sort((a, b) => b.n - a.n)[0] || 0

    for (let i = 0; i < s.length; i++) {
        var [deepLevel, deepLevelEven] = [1, 1];
        while (increaseUneven(i, deepLevel)) {
            unevenMap[i] = unevenMap[i] + 1
            deepLevel++
        }
    
        while (increaseEven(i, deepLevelEven)) {
            evenMap[i] = evenMap[i] + 1
            deepLevelEven++
        }
    }
    
    const {i: unevenIndex, n: unevenOffset} = findChampion(unevenMap)
    const unevenChampion = s.substr(unevenIndex - unevenOffset, unevenOffset * 2 + 1)
    
    const {i: evenIndex, n: evenOffset} = findChampion(evenMap)
    const evenChampion = s.substr(evenIndex - evenOffset + 1, evenOffset * 2)

    return unevenChampion.length > evenChampion.length ? unevenChampion : evenChampion
};