Resolve build errors & cleanup structure (#2334)

Strings/Lower.java

@@ -0,0 +1,28 @@
+package Strings;
+
+public class Lower {
+
+  /** Driver Code */
+  public static void main(String[] args) {
+    String[] strings = {"ABC", "ABC123", "abcABC", "abc123ABC"};
+    for (String s : strings) {
+      assert toLowerCase(s).equals(s.toLowerCase());
+    }
+  }
+
+  /**
+   * Converts all of the characters in this {@code String} to lower case
+   *
+   * @param s the string to convert
+   * @return the {@code String}, converted to lowercase.
+   */
+  public static String toLowerCase(String s) {
+    char[] values = s.toCharArray();
+    for (int i = 0; i < values.length; ++i) {
+      if (Character.isLetter(values[i]) && Character.isUpperCase(values[i])) {
+        values[i] = Character.toLowerCase(values[i]);
+      }
+    }
+    return new String(values);
+  }
+}