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-rw-r--r--src/main/StronglyConnected.java87
1 files changed, 87 insertions, 0 deletions
diff --git a/src/main/StronglyConnected.java b/src/main/StronglyConnected.java
new file mode 100644
index 0000000..adc045b
--- /dev/null
+++ b/src/main/StronglyConnected.java
@@ -0,0 +1,87 @@
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.Scanner;
+import java.util.stream.IntStream;
+
+public class StronglyConnected {
+ private static int numberOfStronglyConnectedComponents(ArrayList<Integer>[] adj) {
+ ArrayList<Integer>[] adjReversa1 = reverseG(adj);
+ boolean[] adjReversalVisited = new boolean[adj.length];
+ int[] postV = new int[adj.length];
+ int[] clock = new int[1];
+ dfs(adjReversa1, adjReversalVisited, postV, clock);
+
+ // https://stackoverflow.com/questions/951848/java-array-sort-quick-way-to-get-a-sorted-list-of-indices-of-an-array/35701049
+ int[] temp = IntStream.range(0, postV.length).boxed().sorted((i, j) -> postV[i] - postV[j]).mapToInt(ele -> ele).toArray();
+
+ boolean[] visited = new boolean[adj.length];
+ int sccCount = 0;
+
+
+ for (int i = adj.length - 1; i > -1; i--) {
+ if (!visited[temp[i]]) {
+ exploreWithoutPostV(temp[i], adj, visited);
+ sccCount++;
+ }
+ }
+ return sccCount;
+ }
+
+ private static void dfs(ArrayList<Integer>[] adj, boolean[] visited, int[] postV, int[] clock) {
+ for (int v = 0; v < adj.length; v++) {
+ if (!visited[v])
+ explore(v, adj, visited, postV, clock);
+ }
+ }
+
+ private static void explore(int v, ArrayList<Integer>[] adj, boolean[] visited, int[] postV, int[] clock) {
+ visited[v] = true;
+ for (int w : adj[v]) {
+ if (!visited[w])
+ explore(w, adj, visited, postV, clock);
+ }
+ postV[v] = clock[0];
+ clock[0] = clock[0] + 1;
+ }
+
+ private static void exploreWithoutPostV(int v, ArrayList<Integer>[] adj, boolean[] visited) {
+ visited[v] = true;
+ for (int w : adj[v]) {
+ if (!visited[w])
+ exploreWithoutPostV(w, adj, visited );
+ }
+ }
+
+
+ private static ArrayList<Integer>[] reverseG(ArrayList<Integer>[] adj) {
+ ArrayList<Integer>[] adjReverse = (ArrayList<Integer>[])new ArrayList[adj.length];
+ for (int i = 0; i < adj.length; i++) {
+ adjReverse[i] = new ArrayList<Integer>();
+ }
+
+ for (int v = 0; v < adj.length; v++) {
+ for (int w : adj[v]) {
+ adjReverse[w].add(v);
+ }
+ }
+ return adjReverse;
+ }
+
+ public static void main(String[] args) {
+ Scanner scanner = new Scanner(System.in);
+ int n = scanner.nextInt();
+ int m = scanner.nextInt();
+ ArrayList<Integer>[] adj = (ArrayList<Integer>[])new ArrayList[n];
+ for (int i = 0; i < n; i++) {
+ adj[i] = new ArrayList<Integer>();
+ }
+ for (int i = 0; i < m; i++) {
+ int x, y;
+ x = scanner.nextInt();
+ y = scanner.nextInt();
+ adj[x - 1].add(y - 1);
+ }
+ System.out.println(numberOfStronglyConnectedComponents(adj));
+ }
+}
+