Replace Words

Problem

Given a string. Replace the words whose length>=4 and is even, with a space between the two equal halves of the word. Consideronly alphabets for finding the evenness of the word
I/P "A person can't walk in this street"
O/P "A per son ca n't wa lk in th is str eet"

Solution

public static String replaceWords(String s) {
    StringBuilder sb = new StringBuilder();
    if (s == null)
        return sb.toString();

    char[] arr = s.toCharArray();
    int n = 0;
    int length = 0;
    while (n < arr.length) {
        if (arr[n] == ' ') {
            if (length >= 4 && (length-1)%2 == 0) {
                sb.append(s.substring(n - length, n - length / 2));
                sb.append(' ');
                sb.append(s.substring(n - length / 2, n));
                length = 0;
            }
            else {
                sb.append(s.substring(n - length, n));
                length = 0;
            }
        }
        length++;
        n++;
    }
    return sb.toString();
}