import itertools

def matchSubstrings(s:str, ss:str, minlen:int=2, reverse:bool=False):
    if reverse: 
        s, ss = ss, s
    ac = itertools.accumulate
    l = len(s)
    if minlen > l: 
        return []
    lll, pos = 0, 0
    for i in range(l - minlen + 1):
        sss = [x for x in ac(s[i:])]
        sss.reverse()
        ll = len(sss)
        for j, x in enumerate(sss[:ll - minlen + 1]):
            if j > ll - lll and i < pos + lll: 
                break
            if x in ss:
                lll = len(x)
                pos = i
                yield x
                break

str1 = "我今天特别开心啊，因为今天是个好日子，我中了 500 万彩票"
str2 = "今天不是个好日子，因为邻居中了 500 万彩票，我今天不开心"

gg = matchSubstrings(str1, str2, 4)
print(next(gg))