Add Kruskal.

README.md

@@ -70,7 +70,7 @@
   * [Bellman-Ford Algorithm](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/graph/bellman-ford) - finding shortest path to all graph vertices
   * [Detect Cycle](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/graph/detect-cycle) - for both directed and undirected graphs (DFS and Disjoint Set based versions)
   * [Prim’s Algorithm](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/graph/prim) - finding Minimum Spanning Tree (MST) for weighted undirected graph
-  * Kruskal’s Algorithm - finding Minimum Spanning Tree (MST)
+  * [Kruskal’s Algorithm](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/graph/kruskal) - finding Minimum Spanning Tree (MST) for weighted undirected graph
   * Topological Sorting
   * Eulerian path, Eulerian circuit
   * Strongly Connected Component algorithm
@@ -85,7 +85,8 @@
 * **Greedy**
   * [Unbound Knapsack Problem](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/sets/knapsack-problem)
   * [Dijkstra Algorithm](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/graph/dijkstra) - finding shortest path to all graph vertices
-  * [Prim’s Algorithm](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/graph/prim) - finding Minimum Spanning Tree (MST)
+  * [Prim’s Algorithm](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/graph/prim) - finding Minimum Spanning Tree (MST) for weighted undirected graph
+  * [Kruskal’s Algorithm](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/graph/kruskal) - finding Minimum Spanning Tree (MST) for weighted undirected graph
 * **Divide and Conquer**
   * [Euclidean Algorithm](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/math/euclidean-algorithm) - calculate the Greatest Common Divisor (GCD)
   * [Permutations](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/sets/permutations) (with and without repetitions)

src/algorithms/graph/kruskal/README.md

@@ -0,0 +1,49 @@
+# Kruskal's Algorithm
+
+Kruskal's algorithm is a minimum-spanning-tree algorithm which 
+finds an edge of the least possible weight that connects any two 
+trees in the forest. It is a greedy algorithm in graph theory 
+as it finds a minimum spanning tree for a connected weighted 
+graph adding increasing cost arcs at each step. This means it 
+finds a subset of the edges that forms a tree that includes every
+vertex, where the total weight of all the edges in the tree is 
+minimized. If the graph is not connected, then it finds a 
+minimum spanning forest (a minimum spanning tree for each 
+connected component).
+
+![Kruskal Algorithm](https://upload.wikimedia.org/wikipedia/commons/5/5c/MST_kruskal_en.gif)
+
+![Kruskal Demo](https://upload.wikimedia.org/wikipedia/commons/b/bb/KruskalDemo.gif)
+
+A demo for Kruskal's algorithm based on Euclidean distance.
+
+## Minimum Spanning Tree
+
+A **minimum spanning tree** (MST) or minimum weight spanning tree 
+is a subset of the edges of a connected, edge-weighted 
+(un)directed graph that connects all the vertices together, 
+without any cycles and with the minimum possible total edge 
+weight. That is, it is a spanning tree whose sum of edge weights 
+is as small as possible. More generally, any edge-weighted 
+undirected graph (not necessarily connected) has a minimum 
+spanning forest, which is a union of the minimum spanning 
+trees for its connected components.
+
+![Minimum Spanning Tree](https://upload.wikimedia.org/wikipedia/commons/d/d2/Minimum_spanning_tree.svg)
+
+A planar graph and its minimum spanning tree. Each edge is 
+labeled with its weight, which here is roughly proportional 
+to its length.
+
+![Minimum Spanning Tree](https://upload.wikimedia.org/wikipedia/commons/c/c9/Multiple_minimum_spanning_trees.svg)
+
+This figure shows there may be more than one minimum spanning 
+tree in a graph. In the figure, the two trees below the graph 
+are two possibilities of minimum spanning tree of the given graph.
+
+## References
+
+- [Minimum Spanning Tree on Wikipedia](https://en.wikipedia.org/wiki/Minimum_spanning_tree)
+- [Kruskal's Algorithm on Wikipedia](https://en.wikipedia.org/wiki/Kruskal%27s_algorithm)
+- [Kruskal's Algorithm on YouTube by Tushar Roy](https://www.youtube.com/watch?v=fAuF0EuZVCk)
+- [Kruskal's Algorithm on YouTube by Michael Sambol](https://www.youtube.com/watch?v=71UQH7Pr9kU)

src/algorithms/graph/kruskal/__test__/kruskal.test.js

@@ -0,0 +1,91 @@
+import GraphVertex from '../../../../data-structures/graph/GraphVertex';
+import GraphEdge from '../../../../data-structures/graph/GraphEdge';
+import Graph from '../../../../data-structures/graph/Graph';
+import kruskal from '../kruskal';
+
+describe('kruskal', () => {
+  it('should fire an error for directed graph', () => {
+    function applyPrimToDirectedGraph() {
+      const graph = new Graph(true);
+
+      kruskal(graph);
+    }
+
+    expect(applyPrimToDirectedGraph).toThrowError();
+  });
+
+  it('should find minimum spanning tree', () => {
+    const vertexA = new GraphVertex('A');
+    const vertexB = new GraphVertex('B');
+    const vertexC = new GraphVertex('C');
+    const vertexD = new GraphVertex('D');
+    const vertexE = new GraphVertex('E');
+    const vertexF = new GraphVertex('F');
+    const vertexG = new GraphVertex('G');
+
+    const edgeAB = new GraphEdge(vertexA, vertexB, 2);
+    const edgeAD = new GraphEdge(vertexA, vertexD, 3);
+    const edgeAC = new GraphEdge(vertexA, vertexC, 3);
+    const edgeBC = new GraphEdge(vertexB, vertexC, 4);
+    const edgeBE = new GraphEdge(vertexB, vertexE, 3);
+    const edgeDF = new GraphEdge(vertexD, vertexF, 7);
+    const edgeEC = new GraphEdge(vertexE, vertexC, 1);
+    const edgeEF = new GraphEdge(vertexE, vertexF, 8);
+    const edgeFG = new GraphEdge(vertexF, vertexG, 9);
+    const edgeFC = new GraphEdge(vertexF, vertexC, 6);
+
+    const graph = new Graph();
+
+    graph
+      .addEdge(edgeAB)
+      .addEdge(edgeAD)
+      .addEdge(edgeAC)
+      .addEdge(edgeBC)
+      .addEdge(edgeBE)
+      .addEdge(edgeDF)
+      .addEdge(edgeEC)
+      .addEdge(edgeEF)
+      .addEdge(edgeFC)
+      .addEdge(edgeFG);
+
+    expect(graph.getWeight()).toEqual(46);
+
+    const minimumSpanningTree = kruskal(graph);
+
+    expect(minimumSpanningTree.getWeight()).toBe(24);
+    expect(minimumSpanningTree.getAllVertices().length).toBe(graph.getAllVertices().length);
+    expect(minimumSpanningTree.getAllEdges().length).toBe(graph.getAllVertices().length - 1);
+    expect(minimumSpanningTree.toString()).toBe('E,C,A,B,D,F,G');
+  });
+
+  it('should find minimum spanning tree for simple graph', () => {
+    const vertexA = new GraphVertex('A');
+    const vertexB = new GraphVertex('B');
+    const vertexC = new GraphVertex('C');
+    const vertexD = new GraphVertex('D');
+
+    const edgeAB = new GraphEdge(vertexA, vertexB, 1);
+    const edgeAD = new GraphEdge(vertexA, vertexD, 3);
+    const edgeBC = new GraphEdge(vertexB, vertexC, 1);
+    const edgeBD = new GraphEdge(vertexB, vertexD, 3);
+    const edgeCD = new GraphEdge(vertexC, vertexD, 1);
+
+    const graph = new Graph();
+
+    graph
+      .addEdge(edgeAB)
+      .addEdge(edgeAD)
+      .addEdge(edgeBC)
+      .addEdge(edgeBD)
+      .addEdge(edgeCD);
+
+    expect(graph.getWeight()).toEqual(9);
+
+    const minimumSpanningTree = kruskal(graph);
+
+    expect(minimumSpanningTree.getWeight()).toBe(3);
+    expect(minimumSpanningTree.getAllVertices().length).toBe(graph.getAllVertices().length);
+    expect(minimumSpanningTree.getAllEdges().length).toBe(graph.getAllVertices().length - 1);
+    expect(minimumSpanningTree.toString()).toBe('A,B,C,D');
+  });
+});

src/algorithms/graph/kruskal/kruskal.js

@@ -0,0 +1,62 @@
+import Graph from '../../../data-structures/graph/Graph';
+import QuickSort from '../../sorting/quick-sort/QuickSort';
+import DisjointSet from '../../../data-structures/disjoint-set/DisjointSet';
+
+/**
+ * @param {Graph} graph
+ * @return {Graph}
+ */
+export default function kruskal(graph) {
+  // It should fire error if graph is directed since the algorithm works only
+  // for undirected graphs.
+  if (graph.isDirected) {
+    throw new Error('Prim\'s algorithms works only for undirected graphs');
+  }
+
+  // Init new graph that will contain minimum spanning tree of original graph.
+  const minimumSpanningTree = new Graph();
+
+  // Sort all graph edges in increasing order.
+  const sortingCallbacks = {
+    /**
+     * @param {GraphEdge} graphEdgeA
+     * @param {GraphEdge} graphEdgeB
+     */
+    compareCallback: (graphEdgeA, graphEdgeB) => {
+      if (graphEdgeA.weight === graphEdgeB.weight) {
+        return 1;
+      }
+
+      return graphEdgeA.weight <= graphEdgeB.weight ? -1 : 1;
+    },
+  };
+  const sortedEdges = new QuickSort(sortingCallbacks).sort(graph.getAllEdges());
+
+  // Create disjoint sets for all graph vertices.
+  const keyCallback = graphVertex => graphVertex.getKey();
+  const disjointSet = new DisjointSet(keyCallback);
+
+  graph.getAllVertices().forEach((graphVertex) => {
+    disjointSet.makeSet(graphVertex);
+  });
+
+  // Go through all edges started from the minimum one and try to add them
+  // to minimum spanning tree. The criteria of adding the edge would be whether
+  // it is forms the cycle or not (if it connects two vertices from one disjoint
+  // set or not).
+  for (let edgeIndex = 0; edgeIndex < sortedEdges.length; edgeIndex += 1) {
+    /** @var {GraphEdge} currentEdge */
+    const currentEdge = sortedEdges[edgeIndex];
+
+    // Check if edge forms the cycle. If it does then skip it.
+    if (!disjointSet.inSameSet(currentEdge.startVertex, currentEdge.endVertex)) {
+      // Unite two subsets into one.
+      disjointSet.union(currentEdge.startVertex, currentEdge.endVertex);
+
+      // Add this edge to spanning tree.
+      minimumSpanningTree.addEdge(currentEdge);
+    }
+  }
+
+  return minimumSpanningTree;
+}