* rename file

* create strings directory
* fix docs
* add test

strings/Alphabetical.java

@@ -0,0 +1,35 @@
+package strings;
+
+/**
+ * <p>
+ * Alphabetical order is a system whereby character strings are placed in order
+ * based on the position of the characters in the conventional ordering of an alphabet.
+ * </p>
+ * Wikipedia: https://en.wikipedia.org/wiki/Alphabetical_order
+ */
+class Alphabetical {
+
+    public static void main(String[] args) {
+        assert !isAlphabetical("123abc");
+        assert isAlphabetical("aBC");
+        assert isAlphabetical("abc");
+        assert !isAlphabetical("xyzabc");
+        assert isAlphabetical("abcxyz");
+    }
+
+    /**
+     * Check if a string is alphabetical order or not
+     *
+     * @param s a string
+     * @return {@code true} if given string is alphabetical order, otherwise {@code false}
+     */
+    public static boolean isAlphabetical(String s) {
+        s = s.toLowerCase();
+        for (int i = 0; i < s.length() - 1; ++i) {
+            if (!Character.isLetter(s.charAt(i)) || !(s.charAt(i) <= s.charAt(i + 1))) {
+                return false;
+            }
+        }
+        return true;
+    }
+}