Graph data structure implementation from scratch, using HashMap

Author: NirmalSilwal

Interview Prep/section20_Graph/Graph.java

@@ -0,0 +1,99 @@
+package section20_Graph;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+
+public class Graph {
+
+	private class Vertex {
+		// stores all neighbors of particular Node
+		HashMap<String, Integer> nbrs = new HashMap<>();
+	}
+
+	HashMap<String, Vertex> vertices;
+
+	public Graph() {
+		vertices = new HashMap<>();
+	}
+
+	public int numVertex() {
+		return this.vertices.size();
+	}
+
+	public boolean containsVertex(String vname) {
+		return this.vertices.containsKey(vname);
+	}
+
+	public void addVertex(String vname) {
+		Vertex vtx = new Vertex();
+		this.vertices.put(vname, vtx);
+	}
+
+	public void removeVertex(String vname) {
+		if (this.vertices.get(vname) == null) {
+			return;
+		}
+		Vertex vtx = this.vertices.get(vname);
+
+		ArrayList<String> allNeighborsKeys = new ArrayList<>(vtx.nbrs.keySet());
+		// removing vname from their neighbors first
+		for (String key : allNeighborsKeys) {
+			Vertex nbrVertex = this.vertices.get(key);
+			nbrVertex.nbrs.remove(vname);
+		}
+		// now removing neighbors form Graph
+		this.vertices.remove(vname);
+	}
+
+	public int numEdges() {
+		int count = 0;
+		ArrayList<String> keys = new ArrayList<>(this.vertices.keySet());
+		for (String key : keys) {
+			Vertex vtx = vertices.get(key);
+			count += vtx.nbrs.size();
+		}
+		return count / 2;
+	}
+
+	public boolean containsEdge(String vname1, String vname2) {
+		Vertex firstNode = vertices.get(vname1);
+		Vertex secondNode = vertices.get(vname2);
+
+		if (firstNode == null || secondNode == null || !firstNode.nbrs.containsKey(vname2)) {
+			return false;
+		}
+		return true;
+	}
+
+	public void addEdge(String vname1, String vname2, int cost) {
+		Vertex vertex1 = vertices.get(vname1);
+		Vertex vertex2 = vertices.get(vname2);
+
+		if (vertex1 == null || vertex2 == null || vertex1.nbrs.containsKey(vname2)) {
+			return;
+		}
+		vertex1.nbrs.put(vname2, cost);
+		vertex2.nbrs.put(vname1, cost);
+	}
+
+	public void removeEdge(String vname1, String vname2) {
+		Vertex vertex1 = vertices.get(vname1);
+		Vertex vertex2 = vertices.get(vname2);
+
+		if (vertex1 == null || vertex2 == null || !vertex1.nbrs.containsKey(vname2)) {
+			return;
+		}
+		vertex1.nbrs.remove(vname2);
+		vertex2.nbrs.remove(vname1);
+	}
+
+	public void dispaly() {
+		System.out.println("--------------------------");
+		ArrayList<String> allKeys = new ArrayList<>(this.vertices.keySet());
+		for (String key : allKeys) {
+			Vertex vtx = this.vertices.get(key);
+			System.out.println(key + ": " + vtx.nbrs);
+		}
+		System.out.println("--------------------------");
+	}
+}