need improve 3. Longest Substring Without Repeating Characters

class Solution {public:    int lengthOfLongestSubstring(string s) {        int b[127]={0};        int i=0;        int sum=0;        int max=0;        for(auto a=s.begin();a!=s.end();a++)        {            if(b[*a-'\0']==0)                b[*a-'\0']=1;            else            {                memset(b,0,sizeof(b));//error                sum=a-s.begin()-i;                if(sum>max) max=sum;                if(a==s.end()-1) return max;                char c=*a;                a--;                while((*a)!=c) a--;                i=a-s.begin()+1;            }        }        sum=s.size()-i;        if(sum>max) max=sum;        return max;    }};