5、最长回文子串

fun longestPalindrome(s: String): String {
    if (s.length < 2) return s
    var strStart = 0    // 最长回文子串起始位置
    var strEnd = 0      // 最长回文子串终止位置
    var startIndex = 0  // 当前子串起始索引
    var endIndex = 0    // 当前子串终止索引
    var nowIndex = 0    // 记录探索的位置
    // 检查回文子串长度
    fun checkLength() {
        while (0 <= startIndex && endIndex < s.length && s[startIndex] == s[endIndex]) {
            if ((strEnd-strStart) < (endIndex - startIndex)) {
                strEnd = endIndex
                strStart = startIndex
            }
            startIndex--
            endIndex++
        }
    }
    while (nowIndex < s.length) {
        // 奇数个回文字符
        startIndex = nowIndex - 1
        endIndex = nowIndex + 1
        checkLength()
        // 偶数个回文字符
        startIndex = nowIndex
        endIndex = nowIndex + 1
        checkLength()
        nowIndex++
    }
    return s.substring(strStart, strEnd+1)
}