CareerCup

public String encode(String input){
        if(null== input) return null;
        input = input.toLowerCase();
        char[] array = input.toCharArray();
        StringBuffer encoded = new StringBuffer(array.length);
        int prevCount=1; char prev = array[0];
        for(int i =1; i < array.length; i++){
            char curr = array[i];
            if(prev == curr){
                prevCount++;
            }else if(prev != curr){
                if(prevCount ==1){
                    encoded.append(prev);
                }else{
                    if(prev=='0' || prev =='1' || prev == '3'|| prev=='4'
                            || prev =='5' || prev =='6'|| prev=='7' || prev =='8' || prev =='9'){
                        encoded.append("_").append(prevCount).append(prev);
                    }else{
                        encoded.append(prevCount).append(prev);
                    }
                }
                prevCount=1;
            }
            prev = curr;
        }
        //for last character
        if(prevCount ==1){
            encoded.append(prev);
        }else{
            if(prev=='0' || prev =='1' || prev == '3'|| prev=='4'
                    || prev =='5' || prev =='6'|| prev=='7' || prev =='8' || prev =='9'){
                encoded.append("_").append(prevCount).append(prev);
            }else{
                encoded.append(prevCount).append(prev);
            }
        }
        return encoded.toString();
    }

Here is a Python implementation:

# wildcad used for digits in string
WILDCARD = "_"


def _compress_n_times(char, count, output):
    """
    Helper to append "char" exactly "count" times to "output".
    """
    if count > 1:
        output += str(count)
    if char.isdigit():
        output += WILDCARD
    output += char
    return output

def compress_string(value):
    """
    Compress a string by using the counts of repeated characters.

    Caveat: adds exactly 1 WILDCARD for each series of digits.
    """
    output = ""
    previous = None
    count = 0
    for char in value:
        if previous is None:
            count = 1
        elif previous == char:
            count += 1
        elif count > 0:
            output = _compress_n_times(previous, count, output)
            count = 1
        previous = char

    if previous is not None:
        if count == 0:
            raise RuntimeError("Reached the end of compression with empty count and previous character.")
        output = _compress_n_times(previous, count, output)

    return output


def decompress_string(value):
    """
    Decompress a string by using the counts of repeated characters.
    """
    output = ""
    count_digits = ""
    previous_was_wildcard = False
    for char in value:
        if char == WILDCARD:
            previous_was_wildcard = True
            continue

        if char.isdigit():
            if previous_was_wildcard:
                output += char * int(count_digits or 1)
                count_digits = ""
                previous_was_wildcard = False
            else:
                count_digits += char
        else:
            output += char * int(count_digits or 1)
            count_digits = ""

    return output