LeetCode之Keyboard Row

import java.util.ArrayList;
public class Solution {
      public ArrayList<String> resultList;
      //检验这个字符串是否全在同一行
      public boolean check(String s1, String s2) {
          if (s1 == null) {
              return false;
          }
          for (int i = 0; i < s1.length(); ++i) {
              if (s2.indexOf(s1.charAt(i)) == -1) {
                  return false;
              }
          }
          return true;
      }
      public  String[] findWords(String[] words) {
          //检验字符串数组是否都为null
          if (words == null) {
              return null;
          }
          int length = words.length;
          String s1 = "qwertyuiop";
          String s2 = "asdfghjkl";
          String s3 = "zxcvbnm";
          resultList = new ArrayList<String>();
          //判断每次
          for (int i = 0; i < length; ++i) {
            String lowerStr = words[i].toLowerCase();
            if (lowerStr != "") {
                  if (s1.indexOf(lowerStr.charAt(0)) != -1) {
                       if(check(lowerStr, s1))
                           resultList.add(words[i]);
                  }
                  if (s2.indexOf(lowerStr.charAt(0)) != -1) {
                   if(check(lowerStr, s2))
                        resultList.add(words[i]);
                }
                  if (s3.indexOf(lowerStr.charAt(0)) != -1) {
                  if(check(lowerStr, s3))
                        resultList.add(words[i]);
                  } 
              }
          }
        String[] strArr = new String[resultList.size()];
        resultList.toArray(strArr);
          return strArr;
      }
}