C++ :: How To Restore Consecutive Enumeration

I have an array of groups of numbers (I call them classes), that might look like this:

Code: static const int arr[] = {1,1,3,3};
std::vector<int> classes( arr , arr + sizeof(arr) / sizeof( arr[ 0 ] ) );

or like this

Code: static const int arr[] = {1,3,3,3,5,5,6,8};
std::vector<int> classes( arr , arr + sizeof(arr) / sizeof( arr[ 0 ] ) );

The groups of numbers are in an increasing order, but not numbered consecutively. So, would I would like to have is this

1,1,2,2

and

1,2,2,2,3,3,4,5

A loop like this

Code: for ( int i = 1; i < classes.size(); i++ )
if ( classes.at( i ) > classes.at( i - 1 ) )
classes.at( i ) = classes.at( i - 1 ) + 1;

does not work, because it changes the preceding value in the group, thus destroying the group. How would you do it?