Longest Substring Without Repeating Characters

class Solution {public:    int lengthOfLongestSubstring(string s) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        if(s=="") return 0;        int len=s.length(),i=0,j=1,res=1;        while(i<len&&j<len){            bool flag=false;            for(int k=i;k<j;k++){                if(s[j]==s[k]){                    i=k+1;                    flag=true;                    break;                }            }            if(!flag) res=j-i+1>res?j-i+1:res;            j++;        }        return res;    }};