Topological Sort

Posted 2020-12-06 johnnyzhao

package _Sort.Algorithm.topological_sort

/**
 * Topological Sort is for Directed Acyclic Graph(DAG,有向无环图)
 * A DAG Graph has least one vertex with in-degree 0 and one vertex with out-degree 0.
 * 入度(in-degree):进入该节点的边的条数
 * 出度(out-degree):从该节点出发的边的条数
 * 1. we need paint a vertex before its adjacent vertex;
 * 2. we can use DFS or BFS
 * */

class DirectedGraphNode constructor(x: Int, neighbors: ArrayList<DirectedGraphNode>) {
    var label = 0
    var neighbors: ArrayList<DirectedGraphNode>? = null

    init {
        this.label = x
        this.neighbors = neighbors
    }
}

class TopologicalSort {
    fun test() {
        val g = Graph(6)
        g.addEdge(5, 2)
        g.addEdge(5, 0)
        g.addEdge(4, 0)
        g.addEdge(4, 1)
        g.addEdge(2, 3)
        g.addEdge(3, 1)
        println("Following is a Topological Sort: ")
        g.topologicalSort()
    }


}

Graph:

package _Sort.Algorithm.topological_sort

import java.util.*
import kotlin.collections.ArrayList

class Graph constructor(v: Int) {
    private var V = 0//number of vertices
    private var adjacents: Array<ArrayList<Int>>? = null

    init {
        this.V = v
        adjacents = Array(V) { ArrayList<Int>() }
        for (i in 0 until V) {
            adjacents!![i] = ArrayList<Int>()
        }
    }

    /**
     * add edge to graph
     * */
    fun addEdge(u: Int, v: Int) {
        if (adjacents != null) {
            adjacents!![u].add(v)
        }
    }

    fun topologicalSort() {
        //create array to store all indegrees of all vertices
        val inDegrees = IntArray(V)

        for (i in 0 until V) {
            val temp = adjacents!![i]
            for (node in temp) {
                inDegrees[node]++
            }
        }

        //create queue and enqueue all vertices with indegree is 0
        val queue = LinkedList<Int>()
        for (i in 0 until V) {
            if (inDegrees[i] == 0) {
                queue.offer(i)
            }
        }
        //count of visited vertex
        var countOfVisited = 0

        val topOrder = ArrayList<Int>()

        //bfs
        while (queue.isNotEmpty()) {
            val cur = queue.poll()
            topOrder.add(cur)
            /*Iterate through all its neighbouring nodes of dequeued node u and decrease
            their in-degree by 1*/
            for (node in adjacents!![cur]) {
                //if in-degree becomes 0, add it into queue
                if (--inDegrees[node] == 0) {
                    queue.add(node)
                }
            }
            countOfVisited++
        }

        //check if there waw cycle
        if (countOfVisited != V) {
            println("There exists a cycle in the graph")
            return
        }

        for (item in topOrder) {
            println(item)
        }
    }
}