def longest_palindrome(s):
    if not s:
        return ""

    start, end = 0, 0

    for i in range(len(s)):
        len1 = expand_around_center(s, i, i)
        len2 = expand_around_center(s, i, i + 1)
        max_len = max(len1, len2)

        if max_len > (end - start):
            start = i - (max_len - 1) // 2
            end   = i + max_len // 2

    return s[start:end + 1]


def expand_around_center(s, left, right):
    while left >= 0 and right < len(s) and s[left] == s[right]:
        left -= 1
        right += 1
    return right - left - 1

s = "babad"
ret = longest_palindrome(s)
print(ret)