Added implementation for detecting cycle in a graph

Algorithms/Graphs/cpp/CycleDetection.cpp

@@ -0,0 +1,139 @@
+
+#include <iostream>
+#include <unordered_map>
+#include <vector>
+using namespace std;
+
+vector<pair<int, int>>* getEdgeSet(unordered_map<int, vector<int>*> &graph, int &edgeCount) {
+
+    // NUMBER OF VERTICES
+    int V = graph.size();
+
+    vector<pair<int, int>> *v = new vector<pair<int, int>>();
+
+    for(int i=0; i<V; i++) {
+        int n = graph[i] -> size();
+        for(int j=0; j<n; j++) {
+            int curr = graph[i]->at(j);
+            if(curr >= i) {
+                v->push_back(make_pair(i, curr));
+            }
+        }
+    }
+
+    edgeCount = v->size();
+    return v;
+
+}
+
+int find(int *parent, int p) {
+    int currParent = p;
+    while(parent[currParent] != -1) {
+        currParent = parent[currParent];
+    }
+    return currParent;
+}
+
+void unionn(int *parent, int p1, int p2) {
+    parent[p2] = p1;
+}
+
+bool detectCycle(unordered_map<int, vector<int>*> &graph) {
+
+    // NUMBER OF VERTICES
+    int V = graph.size();
+
+    // NUMBER OF EDGES
+    int edgeCount = 0;
+
+    // FUNCTION TO GET THE EDGE SET FROM AN ADJACENT LIST
+    vector<pair<int, int>> *edges = getEdgeSet(graph, edgeCount);
+
+    // UTILITY PARENT SET FOR CYCLE DETECTION, INITIALISED WITH -1
+    int *parent = new int[V];
+    for(int i=0; i<V; i++) {
+        parent[i] = -1;
+    }
+
+    for(int i=0; i<edgeCount; i++) {
+
+        // FIND THE PARENT OF BOTH VERTICES OF EDGE
+        int p1 = find(parent, edges->at(i).first);
+        int p2 = find(parent, edges->at(i).second);
+
+        // IF P1 AND P2 ARE SAME THEN BOTH VERTICES LIE IN SAME SET
+        // INCLUDING THIS EDGE RESULTS IN A CYCLE
+        if(p1 == p2)
+            return true;
+
+        // ELSE IF P1 NOT EQUAL TO P2, THEN BOTH VERTICES ARE IN DIFFERENT SETS
+        // INCLUDE BOTH IN SAME SET
+        unionn(parent, p1, p2);
+    }
+
+    // NO CYCLE, RETURN FALSE
+    return false;
+
+
+}
+
+int main() {
+
+    /*
+
+               0
+             /  | \
+            /   |   \
+            1   2    3
+            |   |     \
+            4___5___6   7
+
+    */
+    // INITIAL INPUT FOR NUMBER OF VERTICES AND EDGES
+    int V = 8, E = 8;
+
+    // GRAPH AS ADJACENCY LIST
+    unordered_map<int, vector<int> *> graph;
+    for(int i=0; i<V; i++) {
+        graph[i] = new vector<int>();
+    }
+
+    // INPUT FOR EDGES
+    int **edges = new int*[E];
+    for(int i=0; i<E; i++) {
+        edges[i] = new int[2];
+    }
+
+    edges[0][0] = 0;edges[0][1] = 1;
+    edges[1][0] = 0;edges[1][1] = 2;
+    edges[2][0] = 0;edges[2][1] = 3;
+    edges[3][0] = 1;edges[3][1] = 4;
+    edges[4][0] = 4;edges[4][1] = 5;
+    edges[5][0] = 2;edges[5][1] = 5;
+    edges[6][0] = 5;edges[6][1] = 6;
+    edges[7][0] = 7;edges[7][1] = 3;
+
+    // INPUT FOR NO CYCLE IN GRAPH, MAKE E=7 ABOVE ON LINE 93
+    // edges[0][0] = 0;edges[0][1] = 1;
+    // edges[1][0] = 7;edges[1][1] = 3;
+    // edges[2][0] = 0;edges[2][1] = 3;
+    // edges[3][0] = 1;edges[3][1] = 4;
+    // edges[4][0] = 4;edges[4][1] = 5;
+    // edges[5][0] = 2;edges[5][1] = 5;
+    // edges[6][0] = 5;edges[6][1] = 6;
+
+    //FORMING THE ADJACENCY LIST
+    for(int i=0; i<E; i++) {
+        graph[edges[i][1]] -> push_back(edges[i][0]);
+        graph[edges[i][0]] -> push_back(edges[i][1]);
+    }
+
+    // IF CYCLE EXISTS RETURNS TRUE
+    bool isCycle = detectCycle(graph);
+
+    if(isCycle) 
+        cout << "There is a cycle in the graph\n";
+    else
+        cout << "No cycle in the graph\n";
+
+}