Optimization: don't push forest edges if there are no non-forest edges

This optimizes the process of searching for a replacement edge to refrain from pushing forest edges down when a tree does not have any same-level non-forest edges. If a tree doesn't have any such edges, then there definitely isn't a replacement edge at that level, so we can avoid doing extra work.

Interestingly, with this optimization, ConnGraph reduces to a single Euler tour forest if the graph is a forest, with addEdge and removeEdge taking O(log N) (non-amortized) time with high probability.

pom.xml

@@ -2,7 +2,7 @@
   <modelVersion>4.0.0</modelVersion>
   <groupId>com.github.btrekkie.connectivity</groupId>
   <artifactId>dynamic-connectivity</artifactId>
-  <version>0.1.0</version>
+  <version>0.1.1</version>
   <name>dynamic-connectivity</name>
   <description>Data structure for dynamic connectivity in undirected graphs</description>
   <build>

src/main/java/com/github/btrekkie/connectivity/ConnGraph.java

@@ -829,17 +829,22 @@ public class ConnGraph {
             while (levelVertex1 != null) {
                 EulerTourNode root1 = levelVertex1.arbitraryVisit.root();
                 EulerTourNode root2 = levelVertex2.arbitraryVisit.root();
-                EulerTourNode root;
-                if (root1.size < root2.size) {
-                    root = root1;
-                } else {
-                    root = root2;
-                }
 
-                pushForestEdges(root);
-                replacementEdge = findReplacementEdge(root);
-                if (replacementEdge != null) {
-                    break;
+                // Optimization: if hasGraphEdge is false for one of the roots, then there definitely isn't a
+                // replacement edge at this level
+                if (root1.hasGraphEdge && root2.hasGraphEdge) {
+                    EulerTourNode root;
+                    if (root1.size < root2.size) {
+                        root = root1;
+                    } else {
+                        root = root2;
+                    }
+
+                    pushForestEdges(root);
+                    replacementEdge = findReplacementEdge(root);
+                    if (replacementEdge != null) {
+                        break;
+                    }
                 }
 
                 // To save space, get rid of trees with one node