SOURCE

function longestHuiWenSubString(S) {
    const n = S.length;
    let start = 0, maxLength = 1;
    const dp = new Array(n);
    for (let i = 0;i < n;i++) {
        dp[i] = new Array(n);
        dp[i][i] = true;
    }
    for (let L = 2;L <= n;L++) {
        for (let i = 0;i < n - L + 1;i++) {
            if (S.charAt(i) === S.charAt(i + L - 1)) {
                if (L === 2) dp[i][i + L -1] = true;
                else dp[i][i + L - 1] = dp[i + 1][i + L - 2];
                if (dp[i][i + L - 1]) {
                    if (L > maxLength) {
                        start = i;
                        maxLength = L;
                    }
                }
            } else {
                dp[i][i + L - 1] = false
            }
        }
    }
    return S.slice(start, start + maxLength);
}

console.log(longestHuiWenSubString("azkzab"))