CC150 : String Compression

class Solution {
    public String compress(String str) {
	int size = countCompression(str); //先check compress之后的size，若更大，返回原string。
	if(size >= str.length()) return str;
	StringBuffer sb = new StringBuffer();
	char last = str.charAt(0);
	int count = 1;
	for(int i = 1; i < str.length(); i++) {
	    if(str.charAt(i) == last) {
		count++;
	    } else {
		sb.append(last);
		sb.append(count);
		last = str.charAt(i);
		count = 1;
	    }
	}
	sb.append(last);
	sb.append(count);
	return sb.toString();
    }

    private int countCompression(String str) {
	if(str == null || str.isEmpty()) return 0;
	char last = str.charAt(0);
	int size = 0, count = 1;
	for(int i = 1; i < str.length(); i++) {
	    if(str.charAt(i) == last) {
		count++;
	    } else {
		size += 1 + String.valueOf(count).length();
		last = str.charAt(i);
		count = 1;
	    }
	}
	size += 1 + String.valueOf(count).length();
	return size;
    }
}