Longest substring with no repetitions

This is the code I came up with.It seems correct.

I am interested in:

1) Improvements on code

2) How can it be better than this? i.e. this is O(N^2) Could I have done better in complexity? What should I have read to do it?

Thanks

public int lengthOfLongestSubstringNoRepetitions(String s) {

            if(s == null || s.trim().equals(""))
                return 0;

            Map seen = new HashMap();

            int max = Integer.MIN_VALUE;

            for(int i = 0; i < s.length(); i++){
                for(int j = i; j < s.length(); j++){
                    char c = s.charAt(j);
                    if(seen.containsKey(new Character(c))){
                        //duplicate in substring                    
                        int tmp = j - i;
                        if(max < tmp) max = tmp;
                        break;
                    }
                    else{
                        seen.put(new Character(c),true);
                        if(j == s.length() -1){
                            //Reached the end of string.Add one to get the range
                            if(max < (j - i + 1)) max = (j - i + 1);    
                        }
                    }                
                }
                seen.clear();
            }

            return (max == Integer.MIN_VALUE)?s.length():max;

        }

Here's an O(n) solution. It replaces the Map with a char-indexed vector of the index where each char was last seen.

public static int lengthOfLongestSubstringNoRepetitions(String s) {

    if (s == null)
        return 0;

    // Trimming input even for the non-empty case is more consistent.
    final String str = s.trim();

    if (str.equals(""))
        return 0;

    int seen[] = new int[Character.MAX_VALUE+1];
    for (int i = 0; i  max)
            max = len;
        seen[ch] = j;
    }
    return max;
}

StackExchange Code Review Q#9120, answer score: 5