feat: add kruskal algorithm (TheAlgorithms#137)

* feat: add kruskal algorithm

* Rewrite existing tests and add test for minimum spanning forest

Author: caojoshua

graph/kruskal.ts

@@ -0,0 +1,45 @@
+import { DisjointSet } from '../data_structures/disjoint_set/disjoint_set';
+
+/**
+ * @function kruskal
+ * @description Compute a minimum spanning forest of a weighted undirected graph
+ * @Complexity_Analysis
+ * Time complexity: O(Elog(V))
+ * Space Complexity: O(V)
+ * @param {Edge[]} edges - The edges of the graph
+ * @param {number} num_vertices - The number of vertices in the graph
+ * @return {Edge[], number} - [The edges of the minimum spanning tree, the sum of the weights of the edges in the tree]
+ * @see https://en.wikipedia.org/wiki/Kruskal%27s_algorithm
+ */
+export const kruskal = (edges: Edge[], num_vertices: number): [Edge[], number] => {
+  let cost = 0;
+  let minimum_spanning_tree = [];
+
+  // Use a disjoint set to quickly join sets and find if vertices live in different sets
+  let sets = new DisjointSet(num_vertices);
+
+  // Sort the edges in ascending order by weight so that we can greedily add cheaper edges to the tree
+  edges.sort((a, b) => a.weight - b.weight);
+
+  for (let edge of edges) {
+    if (sets.find(edge.a) !== sets.find(edge.b)) {
+      // Node A and B live in different sets. Add edge(a, b) to the tree and join the nodes' sets together.
+      minimum_spanning_tree.push(edge);
+      cost += edge.weight;
+      sets.join(edge.a, edge.b);
+    }
+  }
+
+  return [minimum_spanning_tree, cost];
+}
+
+export class Edge {
+  a: number = 0;
+  b: number = 0;
+  weight: number = 0;
+  constructor(a: number, b: number, weight: number) {
+    this.a = a;
+    this.b = b;
+    this.weight = weight;
+  }
+}

graph/test/kruskal.test.ts

@@ -0,0 +1,109 @@
+import { Edge, kruskal } from "../kruskal";
+
+let test_graph = (expected_tree_edges: Edge[], other_edges: Edge[], num_vertices: number, expected_cost: number) => {
+  let [tree_edges, cost] = kruskal(expected_tree_edges.concat(other_edges), num_vertices);
+  expect(cost).toStrictEqual(expected_cost);
+  for (let expected_edge of expected_tree_edges) {
+    expect(tree_edges.includes(expected_edge)).toBeTruthy();
+  }
+  for (let unexpected_edge of other_edges) {
+    expect(tree_edges.includes(unexpected_edge)).toBeFalsy();
+  }
+};
+
+
+describe("kruskal", () => {
+
+  it("should return empty tree for empty graph", () => {
+    expect(kruskal([], 0)).toStrictEqual([[], 0]);
+  });
+
+  it("should return empty tree for single element graph", () => {
+    expect(kruskal([], 1)).toStrictEqual([[], 0]);
+  });
+
+  it("should return correct value for two element graph", () => {
+    const edge = new Edge(0, 1, 5);
+    expect(kruskal([edge], 2)).toStrictEqual([[edge], 5]);
+  });
+
+  it("should return the correct value", () => {
+    let expected_tree_edges = [
+      new Edge(0, 1, 1),
+      new Edge(1, 3, 2),
+      new Edge(2, 3, 3),
+    ];
+
+    let other_edges = [
+      new Edge(0, 2, 4),
+      new Edge(0, 3, 5),
+      new Edge(1, 2, 6),
+    ];
+
+    test_graph(expected_tree_edges, other_edges, 7, 6);
+  });
+
+  it("should return the correct value", () => {
+    let expected_tree_edges = [
+      new Edge(0, 2, 2),
+      new Edge(1, 3, 9),
+      new Edge(2, 6, 74),
+      new Edge(2, 7, 8),
+      new Edge(3, 4, 3),
+      new Edge(4, 9, 9),
+      new Edge(5, 7, 5),
+      new Edge(7, 9, 4),
+      new Edge(8, 9, 2),
+    ]
+
+    let other_edges = [
+      new Edge(0, 1, 10),
+      new Edge(2, 4, 47),
+      new Edge(4, 5, 42),
+    ];
+
+    test_graph(expected_tree_edges, other_edges, 10, 116);
+  });
+
+})
+
+describe("kruskal forest", () => {
+  it("should return empty tree for forest of 2 node trees", () => {
+    let edges = [new Edge(0, 1, 10), new Edge(2, 3, 15)];
+    test_graph(edges, [], 4, 25);
+  });
+
+  it("should return the correct value", () => {
+    let expected_tree_edges = [
+      // Tree 1
+      new Edge(0, 2, 2),
+      new Edge(1, 3, 9),
+      new Edge(2, 6, 74),
+      new Edge(2, 7, 8),
+      new Edge(3, 4, 3),
+      new Edge(4, 9, 9),
+      new Edge(5, 7, 5),
+      new Edge(7, 9, 4),
+      new Edge(8, 9, 2),
+
+      // Tree 2
+      new Edge(10, 11, 1),
+      new Edge(11, 13, 2),
+      new Edge(12, 13, 3),
+    ]
+
+    let other_edges = [
+      // Tree 1
+      new Edge(0, 1, 10),
+      new Edge(2, 4, 47),
+      new Edge(4, 5, 42),
+
+      // Tree 2
+      new Edge(10, 12, 4),
+      new Edge(10, 13, 5),
+      new Edge(11, 12, 6),
+    ];
+
+    test_graph(expected_tree_edges, other_edges, 14, 122);
+  });
+});