Longest Substring Without Repeating Characters

Given a string, find the length of the longest substring without repeating characters. For example, the longest substring without repeating letters for "abcabcbb" is "abc", which the length is 3. For "bbbbb" the longest substring is "b", with the length of 1.

class  Solution {
public:
     int lengthOfLongestSubstring( string s) {
         if(s.length()== 0)
             return  0;
            
         int start= 0;
         int max= 0;
         int index[ 256];
         for( int i= 0;i< 256;i++)
            index[i]=- 1;
         int strlength=s.length();
         for( int i= 0;i<strlength;i++)
        {
             // not repeat
             if(index[s[i]]==- 1)
            {
                index[s[i]]=i;
                 if(i-start+ 1>max) max++;
            }
             // repeat
             else
            {
                 // clear hash
                 while(start<=index[s[i]])
                {
                    index[s[start]]=- 1;
                    start++;
                }
                index[s[i]]=i;
            }
        }
         return max;
    }
};