From b8349cfc102edc3964868c6ff3299e6537c36ae1 Mon Sep 17 00:00:00 2001
From: Haidong Ji
Date: Thu, 21 May 2020 21:46:55 -0500
Subject: Check if DAG graph done!

---
 sources/acyclicity.py | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 48 insertions(+)
 create mode 100644 sources/acyclicity.py

(limited to 'sources/acyclicity.py')

diff --git a/sources/acyclicity.py b/sources/acyclicity.py
new file mode 100644
index 0000000..40cdd28
--- /dev/null
+++ b/sources/acyclicity.py
@@ -0,0 +1,48 @@
+# Uses python3
+
+import sys
+
+
+def explore(adj, v, visited, prev, postv, clock):
+    visited.append(v)
+    prev[v] = clock[0]
+    clock[0] = clock[0] + 1
+
+    for n in adj[v]:
+        if n not in visited:
+            explore(adj, n, visited, prev, postv, clock)
+
+    postv[v] = clock[0]
+    clock[0] = clock[0] + 1
+
+
+def dfs(adj, visited, prev, postv, clock):
+    for v in range(len(adj)):
+        if v not in visited:
+            explore(adj, v, visited, prev, postv, clock)
+
+
+def acyclic(adj):
+    visited = []
+    prev = [0 for i in range(len(adj))]
+    postv = [0 for i in range(len(adj))]
+    clock = [0]
+    dfs(adj, visited, prev, postv, clock)
+
+    for u in range(len(adj)):
+        for v in adj[u]:
+            if postv[u] <= postv[v]:
+                return 1
+    return 0
+
+
+if __name__ == '__main__':
+    input = sys.stdin.read()
+    data = list(map(int, input.split()))
+    n, m = data[0:2]
+    data = data[2:]
+    edges = list(zip(data[0:(2 * m):2], data[1:(2 * m):2]))
+    adj = [[] for _ in range(n)]
+    for (a, b) in edges:
+        adj[a - 1].append(b - 1)
+    print(acyclic(adj))
-- 
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