# 1.4: Modeling the Problem¶

## Combinatorial Objects¶

Odds are very good that others have stumbled upon your algorithmic problem before you, perhaps in substantially different contexts. But to find out what is known about your particular "widget optimization problem," you can't hope to look in a book under widget. You must formulate widget optimization in terms of computing properties of common structures such as:

• Permutations -- which are arrangements, or orderings, of items. For example, {1, 4, 3, 2} and {4, 3, 2, 1} are two distinct permutations of the same set of four integers. We have already seen permutations in the robot optimization prob- lem, and in sorting. Permutations are likely the object in question whenever your problem seeks an "arrangement," "tour," "ordering," or "sequence."

• Subsets -- which represent selections from a set of items. For example, {1, 3, 4} and {2} are two distinct subsets of the first four integers. Order does not matter in subsets the way it does with permutations, so the subsets {1, 3, 4} and {4, 3, 1} would be considered identical. We saw subsets arise in the movie scheduling problem. Subsets are likely the object in question whenever your problem seeks a "cluster," "collection," "committee," "group," "packaging," or "selection."

• Trees -- which represent hierarchical relationships between items. Trees are likely the object in question whenever your problem seeks a "hierarchy," "dominance relationship," "ancestor/descendant relationship," or "taxonomy."

• Graphs -- which represent relationships between arbitrary pairs of objects. Graphs are likely the object in question whenever you seek a "network," "circuit," "web," or "relationship."

• Points -- which represent locations in some geometric space. For example, the locations of McDonald's restaurants can be described by points on a map/plane. Points are likely the object in question whenever your problems work on "sites," "positions," "data records," or "locations."

• Polygons -- which represent regions in some geometric spaces. For example, the borders of a country can be described by a polygon on a map/plane. Polygons and polyhedra are likely the object in question whenever you are working on "shapes," "regions," "configurations," or "boundaries."

• Strings -- which represent sequences of characters or patterns. For example, the names of students in a class can be represented by strings. Strings are likely the object in question whenever you are dealing with "text," "charac- ters," "patterns," or "labels."