500. Keyboard Row

Given a List of words, return the words that can be typed using letters of alphabet on only one row’s of American keyboard like the image below.

Example 1:

Input: ["Hello", "Alaska", "Dad", "Peace"]
Output: ["Alaska", "Dad"]

Note:
You may use one character in the keyboard more than once.
You may assume the input string will only contain letters of alphabet.

public class Solution {
    public String[] findWords(String[] words) {
        String[] result = null;

        if (words != null) {
            List<String> list = new ArrayList<String>();
            String rule1 = ".*[^qwertyuiop]+?.*";
            String rule2 = ".*[^asdfghjkl]+?.*";
            String rule3 = ".*[^zxcvbnm]+?.*";
            for (String word : words) {
                String tmp = word.toLowerCase();
                if (!tmp.matches(rule1) || !tmp.matches(rule2) || !tmp.matches(rule3)) {
                    list.add(word);
                }
            }
            result = new String[list.size()];
            for (int i = 0; i < list.size(); i++) {
                result[i] = list.get(i);
            }
        }
        return result;
    }
}