题目链接：467. 环绕字符串中唯一的子字符串
遍历的状态可以用一个字典来描述，字典的键是 a-z 共 26 个字母，值为以该字母开头的最长合法字符串的长度。
from collections import defaultdict
class Solution:
def findSubstringInWraproundString(self, p: str) -> int:
prefix, i = defaultdict(int), 1
def add_prefix(segment: str):
for i in range(min(len(segment), 26)):
prefix[segment[i]] = max(len(segment) - i, prefix[segment[i]])
while i < len(p):
if (ord(p[i]) - ord(p[i - 1]) + 26) % 26 != 1:
add_prefix(p[:i])
p, i = p[i:], 0
i += 1
if p:
add_prefix(p)
return sum(prefix.values())