* 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;
+    }
+}

strings/Palindrome.java

@@ -0,0 +1,68 @@
+package strings;
+
+/**
+ * Wikipedia: https://en.wikipedia.org/wiki/Palindrome
+ */
+class Palindrome {
+
+    /**
+     * Driver Code
+     */
+    public static void main(String[] args) {
+        String[] palindromes = {null, "", "aba", "123321"};
+        for (String s : palindromes) {
+            assert isPalindrome(s) && isPalindromeRecursion(s) && isPalindrome1(s);
+        }
+
+        String[] notPalindromes = {"abb", "abc", "abc123"};
+        for (String s : notPalindromes) {
+            assert !isPalindrome(s) && !isPalindromeRecursion(s) && !isPalindrome1(s);
+        }
+    }
+
+    /**
+     * Check if a string is palindrome string or not
+     *
+     * @param s a string to check
+     * @return {@code true} if given string is palindrome, otherwise {@code false}
+     */
+    public static boolean isPalindrome(String s) {
+        return (s == null || s.length() <= 1) || s.equals(new StringBuilder(s).reverse().toString());
+    }
+
+    /**
+     * Check if a string is palindrome string or not using recursion
+     *
+     * @param s a string to check
+     * @return {@code true} if given string is palindrome, otherwise {@code false}
+     */
+    public static boolean isPalindromeRecursion(String s) {
+        if (s == null || s.length() <= 1) {
+            return true;
+        }
+
+        if (s.charAt(0) != s.charAt(s.length() - 1)) {
+            return false;
+        }
+
+        return isPalindrome(s.substring(1, s.length() - 1));
+    }
+
+    /**
+     * Check if a string is palindrome string or not another way
+     *
+     * @param s a string to check
+     * @return {@code true} if given string is palindrome, otherwise {@code false}
+     */
+    public static boolean isPalindrome1(String s) {
+        if (s == null || s.length() <= 1) {
+            return true;
+        }
+        for (int i = 0, j = s.length() - 1; i < j; ++i, --j) {
+            if (s.charAt(i) != s.charAt(j)) {
+                return false;
+            }
+        }
+        return true;
+    }
+}

strings/ReverseString.java

@@ -0,0 +1,44 @@
+package strings;
+
+/**
+ * Reverse String using different version
+ */
+public class ReverseString {
+
+    public static void main(String[] args) {
+        assert reverse("abc123").equals("321cba");
+        assert reverse2("abc123").equals("321cba");
+    }
+
+    /**
+     * easiest way to reverses the string str and returns it
+     *
+     * @param str string to be reversed
+     * @return reversed string
+     */
+    public static String reverse(String str) {
+        return new StringBuilder(str).reverse().toString();
+    }
+
+    /**
+     * second way to reverses the string str and returns it
+     *
+     * @param str string to be reversed
+     * @return reversed string
+     */
+    public static String reverse2(String str) {
+
+        if (str == null || str.isEmpty()) {
+            return str;
+        }
+        
+        char[] value = str.toCharArray();
+        for (int i = 0, j = str.length() - 1; i < j; i++, j--) {
+            char temp = value[i];
+            value[i] = value[j];
+            value[j] = temp;
+        }
+        return new String(value);
+    }
+
+}