Return the length of the list stored at the specified key. If the key does not exist zero is
returned (the same behaviour as for empty lists). If the value stored at key is not a list an
error is returned.
<p>
Time complexity: O(1)
@param key
@return The length of the list.
Return the length of the list stored at the specified key. If the key does not exist zero is returned (the same behaviour as for empty lists). If the value stored at key is not a list an error is returned. <p> Time complexity: O(1) @param key @return The length of the list.