Encodes a list of strings to a single string.
    :type strs: List[str]
    :rtype: str

def encode(strs):
    """Encodes a list of strings to a single string.
    :type strs: List[str]
    :rtype: str
    """
    res = ''
    for string in strs.split():
        res += str(len(string)) + ":" + string
    return res