Buttercola: Zenefits: Randomize inner word

Buttercola: Zenefits: Randomize inner word
Given a string of words,. Shuffle chars except for the first and last word for each word. 
E.g. Input is  "I want to buy a cup of water"
Output is "I wnat to buy a cup of wtear".

Solution:
For each word, if its length is greater than 3, do shuffling. 

    public String randomizeInnerWord(String s) {

        if (s == null || s.length() == 0) {

            return "";

        }

         

        char[] words = s.toCharArray();

         

        int i = 0;

        int j = 0;

         

        for (i = 0; i < words.length; i++) {

            if (words[i] == ' ') {

                int len = i - j;

                 

                if (len > 3) {

                    shuffle(words, j + 1, i - 2);

                }

                 

                j = i + 1;

            }

        }

         

        if (i - j > 3) {

            shuffle(words, j + 1, i - 2);

        }

         

        return new String(words);

    }

     

    private void shuffle(char[] words, int start, int end) {

        Random generator = new Random();

        for (int i = start + 1; i <= end; i++) {

            int r = generator.nextInt(i - start) + start;

            swap(words, i, r);

        }

    }

     

    private void swap(char[] words, int i, int j) {

        char temp = words[i];

        words[i] = words[j];

        words[j] = temp;

    }

     

    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);

        String input = sc.nextLine();

         

        Solution solution = new Solution();

        String output = solution.randomizeInnerWord(input);

         

        System.out.println(output);

         

        sc.close();

    }

    public char[] toCharArray() {

        // Cannot use Arrays.copyOf because of class initialization order issues

        char result[] = new char[value.length];

        System.arraycopy(value, 0, result, 0, value.length);

        return result;

    }

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