题解 | #密码截取#

while True:
    try:
        s = input()
        L = len(s)
        if L < 2:
            print(L)
        # 初始化二维数组dp
        dp = [[False] * L for _ in range(L)]
        # 单个字符一定是回文
        for i in range(L):
            dp[i][i] = True
        Max = 1
        # 枚举长度，从2开始
        for n in range(2, L+1):
            # 枚举左侧下标i：0～L-1
            for i in range(L):
                # 右侧下标j：j-i+1 = n -> j = n - 1 + i
                j = n - 1 + i
                # 如果右侧下标超出范围，跳出循环
                if j >= L:
                    break
                if s[i] != s[j]:
                    dp[i][j] = False
                else:
                    # 如果子串长度为2或3，一定是回文
                    if j - i < 3:
                        dp[i][j] = True
                    else:
                        dp[i][j] = dp[i+1][j-1]
                # 如果字串回文，判断长度是否大于Max
                if dp[i][j] and j - i + 1 > Max:
                    Max = j - i + 1
        print(Max)
    except:
        break