private int[] solve() {
SearchNode s;
// Symmetry constraint: color[start] = 0
s = branch(new SearchNode(), 0, 0, null);
if (s != null) return s.solution();
else {
int[] cs = new int[V];
for (int v = 0; v < V; v++) cs[v] = -1;
return cs;
}
}
