From a3d28d9ea91f90af072066b2a77030e929ab0f0e Mon Sep 17 00:00:00 2001
From: Haidong Ji
Date: Wed, 20 May 2020 18:37:36 -0500
Subject: Check if DAG (Directed Acyclic Graph) done. In the exercise it's
 Checking Consistency of CS Curriculum. For the clock variable to assign
 pre-visit and post-visit values, I used a trick of defining the clock as
 int[1], so its mutable and I can increment it. I don't know if that's a best
 practice, but for now I'm happy that the program is good. Fun stuff!

---
 src/main/Acyclicity.java     | 59 ++++++++++++++++++++++++++++++++++++++++++++
 src/test/AcyclicityTest.java | 55 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 114 insertions(+)
 create mode 100644 src/main/Acyclicity.java
 create mode 100644 src/test/AcyclicityTest.java

diff --git a/src/main/Acyclicity.java b/src/main/Acyclicity.java
new file mode 100644
index 0000000..1d69ab8
--- /dev/null
+++ b/src/main/Acyclicity.java
@@ -0,0 +1,59 @@
+import java.util.ArrayList;
+import java.util.Scanner;
+
+public class Acyclicity {
+    static int acyclic(ArrayList<ArrayList<Integer>> adj) {
+        ArrayList<Integer> visited = new ArrayList<>();
+        int[] prev = new int[adj.size()];
+        int[] postv = new int[adj.size()];
+        int[] clock = new int[1];
+        clock[0] = 0;
+        dfs(adj, visited, prev, postv, clock);
+
+        for (int u = 0; u < adj.size(); u++) {
+            for (int v : adj.get(u)) {
+                if (postv[u] <= postv[v])
+                    return 1;
+            }
+        }
+        return 0;
+    }
+
+    private static void dfs(ArrayList<ArrayList<Integer>> adj, ArrayList<Integer> visited, int[] prev, int[] postv, int[] clock) {
+        for (int v = 0; v < adj.size(); v++) {
+            if (!visited.contains(v))
+                explore(adj, v, visited, prev, postv, clock);
+        }
+    }
+
+    private static void explore(ArrayList<ArrayList<Integer>> adj, int v, ArrayList<Integer> visited, int[] prev, int[] postv, int[] clock) {
+        visited.add(v);
+        prev[v] = clock[0];
+        clock[0] = clock[0] + 1;
+
+        for (int n : adj.get(v)) {
+            if (!visited.contains(n))
+                explore(adj, n, visited, prev, postv, clock);
+        }
+        postv[v] = clock[0];
+        clock[0] = clock[0] + 1;
+
+    }
+
+    public static void main(String[] args) {
+        Scanner scanner = new Scanner(System.in);
+        int n = scanner.nextInt();
+        int m = scanner.nextInt();
+        ArrayList<ArrayList<Integer>> adj = new ArrayList<ArrayList<Integer>>();
+        for (int i = 0; i < n; i++) {
+            adj.add(new ArrayList<Integer>());
+        }
+        for (int i = 0; i < m; i++) {
+            int x, y;
+            x = scanner.nextInt();
+            y = scanner.nextInt();
+            adj.get(x - 1).add(y - 1);
+        }
+        System.out.println(acyclic(adj));
+    }
+}
diff --git a/src/test/AcyclicityTest.java b/src/test/AcyclicityTest.java
new file mode 100644
index 0000000..9d67894
--- /dev/null
+++ b/src/test/AcyclicityTest.java
@@ -0,0 +1,55 @@
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class AcyclicityTest {
+
+    @Test
+    void test() {
+        ArrayList<ArrayList<Integer>> adj = new ArrayList<>();
+        for (int i = 0; i < 4; i++) {
+            adj.add(new ArrayList<>());
+        }
+
+        adj.get(0).add(1);
+        adj.get(3).add(0);
+        adj.get(1).add(2);
+        adj.get(2).add(0);
+
+        assertEquals(1, Acyclicity.acyclic(adj));
+    }
+
+    @Test
+    void test1() {
+        ArrayList<ArrayList<Integer>> adj = new ArrayList<>();
+        for (int i = 0; i < 5; i++) {
+            adj.add(new ArrayList<>());
+        }
+
+        adj.get(0).add(1);
+        adj.get(1).add(2);
+        adj.get(0).add(2);
+        adj.get(2).add(3);
+        adj.get(0).add(3);
+        adj.get(1).add(4);
+        adj.get(2).add(4);
+
+        assertEquals(0, Acyclicity.acyclic(adj));
+    }
+
+    @Test
+    void test2() {
+        ArrayList<ArrayList<Integer>> adj = new ArrayList<>();
+        for (int i = 0; i < 4; i++) {
+            adj.add(new ArrayList<>());
+        }
+
+        adj.get(0).add(1);
+        adj.get(2).add(1);
+        adj.get(3).add(2);
+
+        assertEquals(0, Acyclicity.acyclic(adj));
+    }
+}
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