Added Kruskal's Algorithm (#232)

Graphs/KruskalMST.js

@@ -0,0 +1,115 @@
+class DisjointSetTreeNode {
+  // Disjoint Set Node to store the parent and rank
+  constructor (key) {
+    this.key = key
+    this.parent = this
+    this.rank = 0
+  }
+}
+
+class DisjointSetTree {
+  // Disjoint Set DataStructure
+  constructor () {
+    // map to from node name to the node object
+    this.map = {}
+  }
+
+  makeSet (x) {
+    // Function to create a new set with x as its member
+    this.map[x] = new DisjointSetTreeNode(x)
+  }
+
+  findSet (x) {
+    // Function to find the set x belongs to (with path-compression)
+    if (this.map[x] !== this.map[x].parent) {
+      this.map[x].parent = this.findSet(this.map[x].parent.key)
+    }
+    return this.map[x].parent
+  }
+
+  union (x, y) {
+    // Function to merge 2 disjoint sets
+    this.link(this.findSet(x), this.findSet(y))
+  }
+
+  link (x, y) {
+    // Helper function for union operation
+    if (x.rank > y.rank) {
+      y.parent = x
+    } else {
+      x.parent = y
+      if (x.rank === y.rank) {
+        y.rank += 1
+      }
+    }
+  }
+}
+
+class GraphWeightedUndirectedAdjacencyList {
+  // Weighted Undirected Graph class
+  constructor () {
+    this.connections = {}
+    this.nodes = 0
+  }
+
+  addNode (node) {
+    // Function to add a node to the graph (connection represented by set)
+    this.connections[node] = {}
+    this.nodes += 1
+  }
+
+  addEdge (node1, node2, weight) {
+    // Function to add an edge (adds the node too if they are not present in the graph)
+    if (!(node1 in this.connections)) { this.addNode(node1) }
+    if (!(node2 in this.connections)) { this.addNode(node2) }
+    this.connections[node1][node2] = weight
+    this.connections[node2][node1] = weight
+  }
+
+  KruskalMST () {
+    // Kruskal's Algorithm to generate a Minimum Spanning Tree (MST) of a graph
+    // Details: https://en.wikipedia.org/wiki/Kruskal%27s_algorithm
+    // getting the edges in ascending order of weights
+    const edges = []
+    const seen = new Set()
+    for (const start of Object.keys(this.connections)) {
+      for (const end of Object.keys(this.connections[start])) {
+        if (!seen.has(`${start} ${end}`)) {
+          seen.add(`${end} ${start}`)
+          edges.push([start, end, this.connections[start][end]])
+        }
+      }
+    }
+    edges.sort((a, b) => a[2] - b[2])
+    // creating the disjoint set
+    const disjointSet = new DisjointSetTree()
+    Object.keys(this.connections).forEach(node => disjointSet.makeSet(node))
+    // MST generation
+    const graph = new GraphWeightedUndirectedAdjacencyList()
+    let numEdges = 0
+    let index = 0
+    while (numEdges < this.nodes - 1) {
+      const [u, v, w] = edges[index]
+      index += 1
+      if (disjointSet.findSet(u) !== disjointSet.findSet(v)) {
+        numEdges += 1
+        graph.addEdge(u, v, w)
+        disjointSet.union(u, v)
+      }
+    }
+    return graph
+  }
+}
+
+function main () {
+  const graph = new GraphWeightedUndirectedAdjacencyList()
+  graph.addEdge(1, 2, 1)
+  graph.addEdge(2, 3, 2)
+  graph.addEdge(3, 4, 1)
+  graph.addEdge(3, 5, 100) // Removed in MST
+  graph.addEdge(4, 5, 5)
+  console.log(graph)
+  console.log(graph.KruskalMST())
+}
+
+main()