SQL & PL/SQL :: Checking For Skipped IDs?

We have a problem where a data load script has caused data in one of our tables to be incorrectly loaded. Instead of loading the parent row, it has loaded an offest that was assigned by the application.

The problems is, we have the following table:

=================================
ID RESULTSET PARENT
=================================
...
100 502 -1
101 502 0
102 502 1
103 502 2
104 502 3
105 503 -1
106 503 0
110 503 1
111 504 0
...
=================================

What's the best approach to use to transform the last column as follows:

=================================
ID RESULTSET PARENT
=================================
...
100 502 99
101 502 100
102 502 101
103 502 102
104 502 103
105 503 104
106 503 105
110 503 106
111 504 110
...
=================================

Note: ID column may not always be ordered sequentially