Returns: A
string vec used as a vector of integer values,
in which each element
vec[i] represents the number of digits
in the group at position
i,
starting with position 0 as the rightmost group
. If
vec.size() <= i,
the number is the same as group
(i - 1);
if
(i < 0 || vec[i] <= 0 || vec[i] == CHAR_MAX),
the size of the digit group is unlimited
.