/** Branch and bound. */
private SearchNode branch(SearchNode s, int v, int color, SearchNode best) {
SearchNode newBest, nextTry;
assert s != null && v >= 0 && color >= 0 && v < V && color < V;
assert best != null || v == 0 && color == 0; // best only null if first call.
if (!s.setColor(v, color)) return null;
if (best != null && s.bound() > best.maxColor()) return best;
if (best == null || s.maxColor() < best.maxColor()) newBest = s;
else newBest = best;
if (!newBest.solved()) {
for (int w = v; w < V; w++) {
for (int nextColor : s.nextColors(w, newBest.maxColor())) {
nextTry = branch(new SearchNode(s), w, nextColor, newBest);
if (nextTry != null && nextTry.maxColor() < newBest.maxColor()) newBest = nextTry;
}
}
}
return newBest;
}
