Leetcode500 Keyboard Row

class Solution {
    public String[] findWords(String[] words) {
        HashSet<Character>[] rows = new HashSet[3];
        ArrayList<String> result = new ArrayList<>();
        String[] letters = {"qwertyuiopQWERTYUIOP","asdfghjklASDFGHJKL","zxcvbnmZXCVBNM"};
        for(int i=0;i<3;i++) {
            rows[i] = new HashSet<Character>();　　//这里容易忽略从而报错nullPointerException
            char[] temp = letters[i].toCharArray();
            for(char l:temp) rows[i].add(l);
        }
        for(String word:words) {
            char[] ls = word.toCharArray();
            int row;
            for(row=0;row<3;row++) if(rows[row].contains(ls[0])) break;
            boolean isOK = true;
            for(char l:ls) if(!rows[row].contains(l)) {isOK=false;break;}
            if(isOK) result.add(word);
        }
        String[] r = new String[result.size()];
        for(int i=0;i<r.length;i++) {
            r[i]=result.get(i);
        }
        return r;
    }
}