  The vertex coloring problem is a well-known NP-hard problem and has many
applications in operations research and in scheduling. A conventional approach
to the problem solves the k-colorability problem iteratively, decreasing k one
by one. Whether a heuristic algorithm finds a legal k-coloring quickly or not
is largely affected by an initial solution. We highlight a simple initial
solution generator, which we call the recycle method, which makes use of the
legal (k+1)-coloring that has been found. An initial solution generated by the
method is expected to guide a heuristic algorithm to find a legal k-coloring
more quickly than conventional methods, as demonstrated by experimental
studies. The results suggest that the recycle method should be used as the
standard initial solution generator for both local search algorithms and modern
hybrid methods.
