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class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        int cnt=s.size(),sum=0;
        map<char,int>mp;
        for(int i=0;i<cnt;i++){
            if(!mp.count(s[i])){
                sum++;
                mp[s[i]]=1;
            }
        }
        if(sum==1)return 1;
        if(sum==2)return 2;
        if(sum==cnt)return sum;

        for(int i=sum;i>=3;i--){
        for(int j=0;j<=cnt-i;j++){//j是当前初始查找位置
            map<char,int>mp2;
            int y=j,sum2=0;
            for(int z=1;z<=i;z++){
                 if(!mp2.count(s[y])){
                sum2++;
                mp2[s[y]]=1;
            }
            y++;
            }
            if(sum2==i)return sum2;
            }
        }
        return 2;
    }
};

https://leetcode.cn/problems/longest-substring-without-repeating-characters/