题解 | 密码截取

# 拓展法

s = input()


def find_substring(chars):

    substring = []  # 存储回文子串

    # 齐数回文子串
    for i in range(len(chars)):
        left = i
        right = i
        while left >= 0 and right < len(chars) and chars[left] == chars[right]:
            substring.append(chars[left : right + 1])  # 存储满足回文子串条件的子串
            left -= 1  # 向外拓展
            right += 1

    # 偶数回文子串
    for j in range(len(chars)):
        left = j
        right = j + 1
        while left >= 0 and right < len(chars) and chars[left] == chars[right]:
            substring.append(chars[left : right + 1])  # 存储满足回文子串条件的子串
            left -= 1
            right += 1

    return set(substring)


sub = find_substring(s)

max_len = max([len(i) for i in sub])

print(max_len)