最长不含重复字符的子字符串

class Solution {
public:
    int longestSubstringWithoutDuplication(string s) {
        int len=0,n=s.size(),cnt[30];
        memset(cnt,0,sizeof cnt);
        for (int i = 0,j = 0; j < n; j ++ )
        {
            cnt[s[j]-'a']++;
            while(i<j&&cnt[s[j]-'a']>1)
            {
                cnt[s[i]-'a']--;
                i++;
            }
            len=max(len,j-i+1);
        }
        return len;
    }
};