<script>
    /**
     * @param {string} s
     * @return {string}
     */
    const longestPalindrome = function (s) {
        if (s.length < 2) {
            return s
        }
        let start = 0, maxLen = 1;
        function help(left, right) {
            // 判断越界情况
            while (left >= 0 && right <= s.length && s[left] == s[right]) {
                if (right - left + 1 > maxLen) {
                    maxLen = right - left + 1
                    start = left
                }
                left--
                right++
            }
        }
        for (let i = 0; i < s.length; i++) {
            // 有中心点的情况 例：cabaa
            help(i - 1, i + 1)
            // 无中心点的情况 例：abba
            help(i, i + 1)
        }
        return s.substring(start, start + maxLen)
    };
</script>