Issue #97 is completed

Author: Hamid Gasmi

3-graph-algorithms/1_graph_decomposition/strongly_connected.py

@@ -2,18 +2,58 @@
 
 sys.setrecursionlimit(200000)
 
-def number_of_strongly_connected_components(adj):
-    result = 0
-    #write your code here
-    return result
+class Graph:
+    def __init__(self, n, edges):
+        self.visited = [False] * n
+        self.adj = [[] for _ in range(n)]
+        self.adjReverse = [[] for _ in range(n)]
+        self.buildAdjacencyList(edges)
+        
+    # Time Complexity: O(|E|)
+    # Space Complexity: O(1)
+    def buildAdjacencyList(self, edges):
+        for (a, b) in edges:
+            self.adj[a - 1].append(b - 1)
+            self.adjReverse[b - 1].append(a - 1)
+
+    def explore(self, v, adj, postOrderVisits):
+        self.visited[v] = True
+        for a in adj[v]:
+            if not self.visited[a]:
+                self.explore(a, adj, postOrderVisits)
+
+        postOrderVisits.append(v)
+
+    def dfs(self, adj, postOrderVisits):
+        self.visited = [False] * n
+        for v in range(len(adj)):
+            if not self.visited[v]:
+                self.explore(v, adj, postOrderVisits)
+
+    def number_of_strongly_connected_components(self):
+        
+        postOrderVisits = []
+        self.dfs(self.adjReverse, postOrderVisits)
+        self.visited = [False] * n
+        aSCCList = []
+        for i in range(len(postOrderVisits) - 1, -1, -1):
+            v = postOrderVisits[i]
+            if not self.visited[v]:
+                aSCC = []
+                self.explore(v, self.adj, aSCC)
+                aSCCList.append(aSCC)
+
+        return aSCCList
+
+def number_of_strongly_connected_components(n, edges):
+    aGraph = Graph(n, edges=edges)
+
+    return len(aGraph.number_of_strongly_connected_components())
 
 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))
+    print(number_of_strongly_connected_components(n, edges))

3-graph-algorithms/1_graph_decomposition/strongly_connected_tests/01r

@@ -0,0 +1 @@
+2

3-graph-algorithms/1_graph_decomposition/strongly_connected_tests/02r

@@ -0,0 +1 @@
+5

3-graph-algorithms/1_graph_decomposition/strongly_connected_tests/03

@@ -0,0 +1,8 @@
+6 7 
+1 2
+2 3
+3 1
+1 4
+4 5
+5 6
+6 4

3-graph-algorithms/1_graph_decomposition/strongly_connected_tests/03e

@@ -0,0 +1 @@
+2

3-graph-algorithms/1_graph_decomposition/strongly_connected_tests/03r

@@ -0,0 +1 @@
+2

3-graph-algorithms/1_graph_decomposition/strongly_connected_tests/04

@@ -0,0 +1,9 @@
+6 7
+1 3
+3 2
+2 1
+6 3
+6 5
+5 4
+4 6
+

3-graph-algorithms/1_graph_decomposition/strongly_connected_tests/04e

@@ -0,0 +1 @@
+2

3-graph-algorithms/1_graph_decomposition/strongly_connected_tests/04r

@@ -0,0 +1 @@
+2

3-graph-algorithms/1_graph_decomposition/strongly_connected_tests/05

@@ -0,0 +1,21 @@
+10 20
+7 8
+4 10
+3 2
+1 3
+4 9
+2 6
+8 3
+8 2
+6 1
+6 10
+10 6
+1 4
+3 8
+1 5
+8 9
+5 3
+3 4
+5 1
+8 5
+8 4

3-graph-algorithms/1_graph_decomposition/strongly_connected_tests/05e

@@ -0,0 +1 @@
+3

3-graph-algorithms/1_graph_decomposition/strongly_connected_tests/05r

@@ -0,0 +1 @@
+3