def tocamelcase(s):
    words = s.split('_')
    camelcase = words[0] + ''.join(word.title() for word in words[1:])
    return camelcase

def tosnakecase(s):
    snakecase = ''
    for char in s:
        if char.isupper():
            snakecase += '_' + char.lower()
        else:
            snakecase += char
    return snakecase.lstrip('_')

# Пример использования
print(tocamelcase("hello_world"))  # Output: helloWorld
print(tosnakecase("helloWorld"))   # Output: hello_world