[Java] Chapter 8. Graph

✔️ Graph(그래프)

• 정점(Vertex)과 간선(Edge)로 이루어짐, 비선형 자료구조 중 하나

1. 그래프의 종류

2. 그래프 구현 방법

• (1) 인접 행렬
• (2) 인접 리스트

인접 행렬 예제

Connection.java

package MatrixGraph;

public class Connection<T> {
    private Node<T> node;
    private int weight;

    public Connection(Node<T> node, int weight) {
        this.node = node;
        this.weight = weight;
    }

    public Node<T> getNode() {
        return node;
    }

    public int getWeight() {
        return weight;
    }
}

Node.java - 제네릭 타입 T 사용

package MatrixGraph;

import java.util.LinkedList;
import java.util.List;
import java.util.Objects;

class Node<T> {
    private T data;
    private List<Connection<T>> links;
    private boolean visited;

    public Node(T name) {
        this.data = name;
        this.links = new LinkedList<>();
    }

    public List<Connection<T>> connections() {
        return links;
    }

    public void link(Node<T> node, int weight) {
        links.add(new Connection(node, weight));
    }

    public void resetVisit() { // visited 초기화 -> 재사용 가능하게 함
        this.visited = false;
    }

    public void visit() {
        this.visited = true;
    }

    public boolean isVisited() {
        return this.visited;
    }

    @Override
    public String toString() {
        return data.toString();
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Node node = (Node) o;
        return Objects.equals(data, node.data);
    }

    @Override
    public int hashCode() {
        return Objects.hash(data);
    }
}

Graph.java - BFS/DFS 함수 내장, 각 탐색 시작하기 전에 rest을 통해 visited 초기화

package MatrixGraph;

import java.util.*;


public class Graph<T> {
    private List<Node<T>> nodes = new ArrayList<>();

    public void addNode(Node<T> node) {
        nodes.add(node);
    }

    public Node<T> getNode(int index) {
        return nodes.get(index);
    }

    public void generate(int[][] matrix) {
        for(int i = 0; i < matrix.length; i++) {
            int[] row = matrix[i];
            for(int j = 0; j < row.length; j++) {
                if(row[j] == 0) continue;
                nodes.get(i).link(nodes.get(j), row[j]);
            }
        }
    }

    public void reset() {
        nodes.forEach(Node::resetVisit);
    }

    public Connection<T> BFS(Node<T> start, Node<T> target) {
        reset();

        Queue<Connection<T>> queue = new LinkedList<>();
        queue.offer(new Connection(start, 0));

        while(!queue.isEmpty()) {
            Connection<T> conn = queue.poll();
            Node<T> n = conn.getNode();
            int weight = conn.getWeight();
            n.visit();

            if(n.equals(target)) {
                return new Connection<>(target, weight);
            }
            n.connections().stream()
                    .filter(i -> !i.getNode().isVisited())
                    .filter(i -> !queue.contains(i))
                    .map(i -> new Connection(i.getNode(), i.getWeight() + weight))
                    .forEach(queue::offer);
        }
        return null;
    }

    public Connection<T> DFS(Node<T> start, Node<T> target) {
        reset();

        Stack<Connection<T>> stack = new Stack<>();
        stack.push(new Connection(start, 0));

        while(!stack.isEmpty()) {
            Connection<T> conn = stack.pop();
            Node<T> n = conn.getNode();
            int weight = conn.getWeight();
            n.visit();

            if(n.equals(target)) {
                return new Connection<T>(target, weight);
            }
            n.connections().stream()
                    .filter(i -> !i.getNode().isVisited())
                    .filter(i -> !stack.contains(i))
                    .map(i -> new Connection(i.getNode(), i.getWeight() + weight))
                    .forEach(stack::push);
        }
        return null;
    }
}

Main.java

package MatrixGraph;

public class Main {
    public static void main(String[] args) {
        Graph<String> graph = new Graph<String>();
        graph.addNode(new Node("A"));
        graph.addNode(new Node("B"));
        graph.addNode(new Node("C"));
        graph.addNode(new Node("D"));
        graph.addNode(new Node("E"));

        graph.generate(new int[][] {
                { 0, 0, 0, 2, 0 },
                { 5, 0, 6, 0, 4 },
                { 0, 6, 0, 0, 0 },
                { 0, 0, 2, 0, 0 },
                { 0, 0, 0, 3, 0 }
        });

        Node a = graph.getNode(0);
        Node target = graph.getNode(4);

        var answer1 = graph.BFS(a, target);
        System.out.println(String.format("BFS : %s (%d)", answer1.getNode(), answer1.getWeight())); // BFS : E(14)

        var answer2 = graph.DFS(a, target);
        System.out.println(String.format("DFS : %s (%d)", answer2.getNode(), answer2.getWeight())); // DFS : E(14)
    }
}

3. 인접 행렬과 인접 리스트 비교