给定一个字符串，找出没有重复字符的最长的子串。
def longest_substr(s):dic = {}start,maxlen,substr = 0,0,""for i,x in enumerate(s):if x in dic:start = max(dic[x]+1,start)dic[x] = ielse:dic[x] = iif i-start+1>maxlen:maxlen = i-start+1substr = s[start:i+1]return(substr)print(longest_substr("abcbefdsafgf"))
print(longest_substr("abdsgcdhef"))
print(longest_substr("abbacsafdefh"))