Help with an algorithm...

[Paul Dupuis]

Today is not my coding day. I have a problem I should be able to design 
a solution for an am struggling. Clearly I am missing "something"

I have 2 lists (LISTNEW and LISTOLD) of the following format:

ParentA
<space>Child 1
<space>Child 2
<space>etc.
ParentB
<space>Child 1
<space>etc.
etc.

The parents are in alphabetical sorted order, the children may not be in 
sorted order

I need to hunt through LISTOLD comparing the LISTOLD Parents to the 
LISTNEW Parents
   FOR any LISTOLD Parent present in LISTNEW, check the Children of the 
matching Parents and add any Child for the LISTOLD Parent that is not 
already under its matching LISTNEW Parent
   FOR any LISTOLD Parent NOT in LISTNEW, I can ignore the Parent and 
its Children

I can not seem to write an approach to solve this today. Does any body 
have some code so solve this they may be willing to share?