387. First Unique Character in a String

public class Solution {
    public int firstUniqChar(String s) {
        
        if (s==null)
        return -1;
        Set<Integer> tmp=new HashSet<Integer>();
        int[]res=new int[26];
        char[] sToChar=s.toCharArray();
        for(int i=0; i<s.length();i++){
            res[(sToChar[i]-'a')]++;
        }
        for(int i=0;i<26;i++){
            if(res[i]>=2)
            tmp.add(i);
            
        }
        for(int i=0;i<s.length();i++){
            if(tmp.add(sToChar[i]-'a'))
            return i;
        }
        return -1;
    }
}