题解 | 密码截取

import sys

# 读取字符串，去掉末尾换行
s = sys.stdin.readline().strip()
n = len(s)
max_len = 1  # 最短单个字符也是回文，长度为1

# 遍历每一个位置，当做回文中心
for i in range(n):
    # ========== 情况1：奇数长度回文，中心是单个字符 ==========
    l, r = i, i
    while l >= 0 and r < n and s[l] == s[r]:
        # 左右相等，向外扩散
        current = r - l + 1
        max_len = max(max_len, current)
        l -= 1
        r += 1

    # ========== 情况2：偶数长度回文，中心是两个相邻字符 ==========
    l, r = i, i+1
    while l >= 0 and r < n and s[l] == s[r]:
        current = r - l + 1
        max_len = max(max_len, current)
        l -= 1
        r += 1

# 输出最长回文长度
print(max_len)