graphs/kruskal: add a test case to verify the correctness, fix styles (#2443)

* test/graphs/kruskal: adding a test case to verify the correctness of the algorithm

Fixes #2128

* grahps/kruskal: running psf/black

* graphs/kruskal: read edges in a friendlier fashion

Co-authored-by: Christian Clauss <cclauss@me.com>

* Update minimum_spanning_tree_kruskal.py

* fixup! Format Python code with psf/black push

* Update test_min_spanning_tree_kruskal.py

* updating DIRECTORY.md

Co-authored-by: Christian Clauss <cclauss@me.com>
Co-authored-by: John Law <johnlaw.po@gmail.com>
Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>

DIRECTORY.md

@@ -293,6 +293,8 @@
   * [Scc Kosaraju](https://github.com/TheAlgorithms/Python/blob/master/graphs/scc_kosaraju.py)
   * [Strongly Connected Components](https://github.com/TheAlgorithms/Python/blob/master/graphs/strongly_connected_components.py)
   * [Tarjans Scc](https://github.com/TheAlgorithms/Python/blob/master/graphs/tarjans_scc.py)
+  * Tests
+    * [Test Min Spanning Tree Kruskal](https://github.com/TheAlgorithms/Python/blob/master/graphs/tests/test_min_spanning_tree_kruskal.py)
 
 ## Greedy Method
   * [Greedy Knapsack](https://github.com/TheAlgorithms/Python/blob/master/greedy_method/greedy_knapsack.py)

graphs/minimum_spanning_tree_kruskal.py

@@ -1,13 +1,5 @@
-if __name__ == "__main__":
-    num_nodes, num_edges = list(map(int, input().strip().split()))
-
-    edges = []
-
-    for i in range(num_edges):
-        node1, node2, cost = list(map(int, input().strip().split()))
-        edges.append((i, node1, node2, cost))
-
-    edges = sorted(edges, key=lambda edge: edge[3])
+def kruskal(num_nodes, num_edges, edges):
+    edges = sorted(edges, key=lambda edge: edge[2])
 
     parent = list(range(num_nodes))
 
@@ -20,13 +12,22 @@ if __name__ == "__main__":
     minimum_spanning_tree = []
 
     for edge in edges:
-        parent_a = find_parent(edge[1])
-        parent_b = find_parent(edge[2])
+        parent_a = find_parent(edge[0])
+        parent_b = find_parent(edge[1])
         if parent_a != parent_b:
-            minimum_spanning_tree_cost += edge[3]
+            minimum_spanning_tree_cost += edge[2]
             minimum_spanning_tree.append(edge)
             parent[parent_a] = parent_b
 
-    print(minimum_spanning_tree_cost)
-    for edge in minimum_spanning_tree:
-        print(edge)
+    return minimum_spanning_tree
+
+
+if __name__ == "__main__":  # pragma: no cover
+    num_nodes, num_edges = list(map(int, input().strip().split()))
+    edges = []
+
+    for _ in range(num_edges):
+        node1, node2, cost = [int(x) for x in input().strip().split()]
+        edges.append((node1, node2, cost))
+
+    kruskal(num_nodes, num_edges, edges)

graphs/tests/test_min_spanning_tree_kruskal.py

@@ -0,0 +1,36 @@
+from graphs.minimum_spanning_tree_kruskal import kruskal
+
+
+def test_kruskal_successful_result():
+    num_nodes, num_edges = 9, 14
+    edges = [
+        [0, 1, 4],
+        [0, 7, 8],
+        [1, 2, 8],
+        [7, 8, 7],
+        [7, 6, 1],
+        [2, 8, 2],
+        [8, 6, 6],
+        [2, 3, 7],
+        [2, 5, 4],
+        [6, 5, 2],
+        [3, 5, 14],
+        [3, 4, 9],
+        [5, 4, 10],
+        [1, 7, 11],
+    ]
+
+    result = kruskal(num_nodes, num_edges, edges)
+
+    expected = [
+        [7, 6, 1],
+        [2, 8, 2],
+        [6, 5, 2],
+        [0, 1, 4],
+        [2, 5, 4],
+        [2, 3, 7],
+        [0, 7, 8],
+        [3, 4, 9],
+    ]
+
+    assert sorted(expected) == sorted(result)