Leetcode: Longest Substring Without Repeating Characters

int lengthOfLongestSubstring(string s) {  
        // Start typing your C/C++ solution below  
        // DO NOT write int main() function  
        int longest = 0;  
        int slen = s.length();  
        if(slen < 0) return 0;  
        if(slen == 1) return 1;  
        int pre = 1;  
        for(int i = 1; i < slen; ++i)  
        {  
            char tmp = s[i];  
            int j = i-1;  
            int count = 1;  
            while((j>=0) && (tmp != s[j]))  
            {j--;count++;}  
            if(count >= pre+1) count = pre+1;  
            if(count > longest) longest = count;  
            pre = count;  
        }  
        return longest;  
    }