获取字符串中重复字符之间最长的子字符串

def solution(s):
    max_len=0
    f=0
    l=0
    s_x=set(s)
    for i in s_x:
        first_d=s.find(i)
        last_d=s.rfind(i)
        if(first_d==last_d):
            pass
        else:
            distans=last_d-first_d
            if(distans>max_len):
                f=first_d
                l=last_d
                max_len=distans
    if(l==(len(s)-1)):
        l=None
    else:
        l=l+1
    return s[f:l]
        
s="abcdaaaa"
print(solution(s))