Cycle issue fixed

Author: karthi1953

src/algorithms/graph/detect-cycle/detectUndirectedCycleUsingDisjointSet.js

@@ -1,31 +1,64 @@
 import DisjointSet from '../../../data-structures/disjoint-set/DisjointSet';
 
 /**
- * Detect cycle in undirected graph using disjoint sets.
- *
+ * Detect and return the actual cycle path in an undirected graph using disjoint sets.
  * @param {Graph} graph
+ * @returns {Array|null} - Returns an array of vertex keys forming the cycle, or null if no cycle found.
  */
 export default function detectUndirectedCycleUsingDisjointSet(graph) {
-  // Create initial singleton disjoint sets for each graph vertex.
-  /** @param {GraphVertex} graphVertex */
   const keyExtractor = (graphVertex) => graphVertex.getKey();
   const disjointSet = new DisjointSet(keyExtractor);
-  graph.getAllVertices().forEach((graphVertex) => disjointSet.makeSet(graphVertex));
-
-  // Go trough all graph edges one by one and check if edge vertices are from the
-  // different sets. In this case joint those sets together. Do this until you find
-  // an edge where to edge vertices are already in one set. This means that current
-  // edge will create a cycle.
-  let cycleFound = false;
-  /** @param {GraphEdge} graphEdge */
-  graph.getAllEdges().forEach((graphEdge) => {
-    if (disjointSet.inSameSet(graphEdge.startVertex, graphEdge.endVertex)) {
-      // Cycle found.
-      cycleFound = true;
-    } else {
-      disjointSet.union(graphEdge.startVertex, graphEdge.endVertex);
-    }
+  const parentMap = new Map();
+
+  graph.getAllVertices().forEach((vertex) => {
+    disjointSet.makeSet(vertex);
+    parentMap.set(vertex.getKey(), null); // Initialize with no parent
   });
 
-  return cycleFound;
+  for (const edge of graph.getAllEdges()) {
+    const startKey = edge.startVertex.getKey();
+    const endKey = edge.endVertex.getKey();
+
+    if (disjointSet.inSameSet(edge.startVertex, edge.endVertex)) {
+      // Cycle detected: reconstruct cycle path
+      return constructCyclePath(startKey, endKey, parentMap);
+    }
+
+    // Save parent info (arbitrarily choosing one as child)
+    parentMap.set(endKey, startKey);
+    disjointSet.union(edge.startVertex, edge.endVertex);
+  }
+
+  return null;
+}
+
+/**
+ * Construct an ordered cycle path using parent map.
+ * @param {string} startKey
+ * @param {string} endKey
+ * @param {Map} parentMap
+ * @returns {string[]} Ordered array of vertex keys forming a cycle
+ */
+function constructCyclePath(startKey, endKey, parentMap) {
+  const pathToRoot = (key) => {
+    const path = [];
+    while (key !== null) {
+      path.push(key);
+      key = parentMap.get(key);
+    }
+    return path;
+  };
+
+  const pathStart = pathToRoot(startKey);
+  const pathEnd = pathToRoot(endKey);
+
+  // Find the last common ancestor
+  const setStart = new Set(pathStart);
+  const commonAncestor = pathEnd.find((key) => setStart.has(key));
+
+  // Slice paths up to the common ancestor
+  const cycleStart = pathStart.slice(0, pathStart.indexOf(commonAncestor) + 1);
+  const cycleEnd = pathEnd.slice(0, pathEnd.indexOf(commonAncestor)).reverse();
+
+  return [...cycleStart, ...cycleEnd, commonAncestor];
 }