Longest Substring Without Repeating Characters

int longestSubString(char* arr, int len)
{
    if (arr == NULL || len <= 0)
        return 0;
    int i = 0, j = 0;
    int maxLen = 0;
    bool exist[256] = { false };
    while (j < len)
    {
        if (exist[s[j]])
        {
            maxLen = max(maxLen, j-i);
            while (s[i] != s[j])
            {
                exist[s[i]] = false;
                i++;
            }
            i++;
            j++;
        }
        else
        {
            exist[s[j]] = true;
            j++;
        }
    }
    maxLen = max(maxLen, len-i);
    return maxLen;
}