d23p2 use pivoting

day23/d23p2.swift

@@ -16,13 +16,14 @@ func readInput(_ filePath: String) throws -> Network {
 func bronkerbosch(
     _ network: Network, r: Set<String>, p: Set<String>, x: Set<String>
 ) -> Set<String>? {
-    if p.count == 0 && x.count == 0 {
-        return r
+    if p.count == 0 {
+        return x.count == 0 ? r : nil
     }
     var maxClique: Set<String>? = nil
     var nextP = p
     var nextX = x
-    for v in p {
+    let u = p.union(x).first!  // can also choose highest degree node
+    for v in p.subtracting(network[u]!) {
         if let clique = bronkerbosch(network,
             r: r.union([v]),
             p: nextP.intersection(network[v]!),