Level structure
{{#invoke:Hatnote|hatnote}} In the mathematical subfield of graph theory a level structure of a rooted graph is a partition of the vertices into subsets that have the same distance from a given root vertex.<ref name="dps">Template:Cite FTP.</ref>
Definition and construction
Given a connected graph G = (V, E) with V the set of vertices and E the set of edges, and with a root vertex r, the level structure is a partition of the vertices into subsets Li called levels, consisting of the vertices at distance i from r. Equivalently, this set may be defined by setting L0 = {r}, and then, for i > 0, defining Li to be the set of vertices that are neighbors to vertices in Li − 1 but are not themselves in any earlier level.<ref name="dps"/>
The level structure of a graph can be computed by a variant of breadth-first search:<ref name="mehlhorn">Template:Cite book</ref>Template:Rp
algorithm level-BFS(G, r):
Q ← {r}
for ℓ from 0 to ∞:
process(Q, ℓ) // the set Q holds all vertices at level ℓ
mark all vertices in Q as discovered
Q' ← {}
for u in Q:
for each edge (u, v):
if v is not yet marked:
add v to Q'
if Q' is empty:
return
Q ← Q'
Properties
In a level structure, each edge of G either has both of its endpoints within the same level, or its two endpoints are in consecutive levels.<ref name="dps"/>
Applications
The partition of a graph into its level structure may be used as a heuristic for graph layout problems such as graph bandwidth.<ref name="dps"/> The Cuthill–McKee algorithm is a refinement of this idea, based on an additional sorting step within each level.<ref>Template:Citation.</ref>
Level structures are also used in algorithms for sparse matrices,<ref>Template:Citation.</ref> and for constructing separators of planar graphs.<ref>Template:Citation.</ref>