A method includes accessing data representing a layout of a layer of an integrated circuit (IC) having a plurality of polygons defining circuit patterns to be divided among a number (N) of photomasks over a single layer of a semiconductor substrate, where N is greater than two. The method further includes inputting a conflict graph having a plurality of vertices, identifying a first and second vertex, each of which is connected to a third and fourth vertex where the third and fourth vertices are connected to a same edge of a conflict graph, and merging the first and second vertices to form a reduced graph. The method further includes detecting at least one or more vertex in the reduced having a conflict. In one aspect, the method resolves the detected conflict by performing one of pattern shifting, stitch inserting, or re-routing.