From 7e295af56cffda1dc0a65e3f905e58509dcafebc Mon Sep 17 00:00:00 2001
From: Haidong Ji
Date: Wed, 15 Jul 2020 20:03:51 -0500
Subject: Strongly connected components done!

---
 sources/strongly_connected.py | 66 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 66 insertions(+)
 create mode 100644 sources/strongly_connected.py

(limited to 'sources')

diff --git a/sources/strongly_connected.py b/sources/strongly_connected.py
new file mode 100644
index 0000000..c81725d
--- /dev/null
+++ b/sources/strongly_connected.py
@@ -0,0 +1,66 @@
+# Uses python3
+
+import sys
+
+sys.setrecursionlimit(200000)
+
+
+def explore(v, adj, visited, postV, clock):
+    visited[v] = True
+    for w in adj[v]:
+        if visited[w] == False:
+            explore(w, adj, visited, postV, clock)
+    postV[v] = clock[0]
+    clock[0] = clock[0] + 1
+
+
+def reverseG(adj):
+    adjReverse = [[] for _ in range(len(adj))]
+    for v in range(len(adj)):
+        for w in adj[v]:
+            adjReverse[w].append(v)
+
+    return adjReverse
+
+
+def dfs(adj, visited, postV, clock):
+    for v in range(len(adj)):
+        if visited[v] == False:
+            explore(v, adj, visited, postV, clock)
+
+
+def exploreWithoutPostV(v, adj, visited):
+    visited[v] = True
+    for w in adj[v]:
+        if visited[w] == False:
+            exploreWithoutPostV(w, adj, visited)
+
+
+def number_of_strongly_connected_components(adj):
+    adjReversal = reverseG(adj)
+    adjReversalVisited = [False] * len(adj)
+    postV = [0] * len(adj)
+    clock = [0]
+
+    dfs(adjReversal, adjReversalVisited, postV, clock)
+    temp = sorted(range(len(postV)), key=lambda k: postV[k], reverse=True)
+
+    visited = [False] * len(adj)
+    result = 0
+    for i in temp:
+        if visited[i] == False:
+            exploreWithoutPostV(i, adj, visited)
+            result = result + 1
+    return result
+
+
+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(number_of_strongly_connected_components(adj))
-- 
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