CareerCup 150

C.C 1.5

/**
 * 1.5 Implement a method to perform basic string compression using the counts
 * of repeated characters. For example, the string aabcccccaaa would become
 * a2blc5a3. If the "compressed" string would not become smaller than the
 * original string, your method should return the original string.
 *
 * @author Jiateng
 */
public class CC1_5 {

    public static String compression(String target) {

        if (target == null || target.isEmpty() || target.length() <= 2) {
            return target;
        }

        char[] array = target.toCharArray();
        StringBuilder result = new StringBuilder();

        int count = 1;
        char prev = target.charAt(0);
        char cur;
        for (int i = 1; i < array.length; i++) {
            cur = target.charAt(i);
            if (cur == prev) {
                count++;
            } else {
                result.append(prev);
                result.append(count);
                prev = cur;
                count = 1;
            }
        }
        //add the last character and count
        result.append(prev);
        result.append(count);
        //compare the length
        return result.length() < array.length ? result.toString() : new String(array);
    }

    public static void main(String[] args) {
        System.out.println(compression("abcdefg"));
        System.out.println(compression("aaabbbbddddiiiifffegnnnnn"));
    }
}

1. target.isEmpty() is not necessary since you have checked target.length() <= 2. (Line 13)
2. You can reach your goal without using StringBuilder. Have a try.