python - Clustering polygons by sum of values

I would like to group polygons that (1) are contiguous and (2) when the combined values of field X >= Y. Specifically I am grouping zipcodes that are connected. a group is when 1 or more zipcodes 'population' field equals 100,000.

Can someone suggest an existing ArcGIS function or python method for doing this?

Many thanks in advance!

---update----

I wrote a python algorithm to group the polygons using their respective neighbors as grouping members until threshold is met. it works nicely.