文章目录
- 图
- 1) 概念
- 有向 vs 无向
- 度
- 权
- 路径
- 环
- 图的连通性
- 2) 图的表示
- 3) Java 表示
- 4) DFS
- 5) BFS
- 6) 拓扑排序
- 7) 最短路径
- Dijkstra
- Bellman-Ford
- Floyd-Warshall
- 8) 最小生成树
- Prim
- Kruskal
图
1) 概念
图是由顶点(vertex)和边(edge)组成的数据结构,例如
该图有四个顶点:A、B、C、D 以及四条有向边,有向图中,边是单向的
有向 vs 无向
如果是无向图,那么边是双向的,下面是一个无向图的例子
度
度是指与该顶点相邻的边的数量
例如上图中
- A、B、C、E、F 这几个顶点度数为 2
- D 顶点度数为 4
有向图中,细分为入度和出度,参见下图
- A (2 out / 0 in)
- B、C、E (1 out / 1 in)
- D (2 out / 2 in)
- F (0 out / 2 in)
权
边可以有权重,代表从源顶点到目标顶点的距离、费用、时间或其他度量。
路径
路径被定义为从一个顶点到另一个顶点的一系列连续边,例如上图中【北京】到【上海】有多条路径
- 北京 - 上海
- 北京 - 武汉 - 上海
路径长度
- 不考虑权重,长度就是边的数量
- 考虑权重,一般就是权重累加
环
在有向图中,从一个顶点开始,可以通过若干条有向边返回到该顶点,那么就形成了一个环
图的连通性
如果两个顶点之间存在路径,则这两个顶点是连通的,所有顶点都连通,则该图被称之为连通图,若子图连通,则称为连通分量
2) 图的表示
比如说,下面的图
用邻接矩阵可以表示为:
A B C D
A 0 1 1 0
B 1 0 0 1
C 1 0 0 1
D 0 1 1 0
用邻接表可以表示为:
A -> B -> C
B -> A -> D
C -> A -> D
D -> B -> C
有向图的例子
A B C D
A 0 1 1 0
B 0 0 0 1
C 0 0 0 1
D 0 0 0 0
A - B - C
B - D
C - D
D - empty
3) Java 表示
顶点
public class Vertex {String name;List<Edge> edges;// 拓扑排序相关int inDegree;int status; // 状态 0-未访问 1-访问中 2-访问过,用在拓扑排序// dfs, bfs 相关boolean visited;// 求解最短距离相关private static final int INF = Integer.MAX_VALUE;int dist = INF;Vertex prev = null;@Overridepublic String toString() {return this.name;}
}
边
public class Edge {Vertex linked;int weight;public Edge(Vertex linked) {this(linked, 1);}public Edge(Vertex linked, int weight) {this.linked = linked;this.weight = weight;}
}
4) DFS
public class Dfs {public static void main(String[] args) {Vertex v1 = new Vertex("v1");Vertex v2 = new Vertex("v2");Vertex v3 = new Vertex("v3");Vertex v4 = new Vertex("v4");Vertex v5 = new Vertex("v5");Vertex v6 = new Vertex("v6");v1.edges = List.of(new Edge(v3), new Edge(v2), new Edge(v6));v2.edges = List.of(new Edge(v4));v3.edges = List.of(new Edge(v4), new Edge(v6));v4.edges = List.of(new Edge(v5));v5.edges = List.of();v6.edges = List.of(new Edge(v5));dfs1(v1);}private static void dfs2(Vertex v) {LinkedList<Vertex> stack = new LinkedList<>();stack.push(v);while (!stack.isEmpty()) {Vertex pop = stack.pop();pop.visited = true;System.out.println(pop.name);for (Edge edge : pop.edges) {if (!edge.linked.visited) {stack.push(edge.linked);}}}}private static void dfs1(Vertex v) {v.visited = true;System.out.println(v.name);for (Edge edge : v.edges) {if (!edge.linked.visited) {dfs(edge.linked);}}}
}
5) BFS
public class Bfs {public static void main(String[] args) {Vertex v1 = new Vertex("v1");Vertex v2 = new Vertex("v2");Vertex v3 = new Vertex("v3");Vertex v4 = new Vertex("v4");Vertex v5 = new Vertex("v5");Vertex v6 = new Vertex("v6");v1.edges = List.of(new Edge(v3), new Edge(v2), new Edge(v6));v2.edges = List.of(new Edge(v4));v3.edges = List.of(new Edge(v4), new Edge(v6));v4.edges = List.of(new Edge(v5));v5.edges = List.of();v6.edges = List.of(new Edge(v5));bfs(v1);}private static void bfs(Vertex v) {LinkedList<Vertex> queue = new LinkedList<>();v.visited = true;queue.offer(v);while (!queue.isEmpty()) {Vertex poll = queue.poll();System.out.println(poll.name);for (Edge edge : poll.edges) {if (!edge.linked.visited) {edge.linked.visited = true;queue.offer(edge.linked);}}}}
}
6) 拓扑排序
public class TopologicalSort {public static void main(String[] args) {Vertex v1 = new Vertex("网页基础");Vertex v2 = new Vertex("Java基础");Vertex v3 = new Vertex("JavaWeb");Vertex v4 = new Vertex("Spring框架");Vertex v5 = new Vertex("微服务框架");Vertex v6 = new Vertex("数据库");Vertex v7 = new Vertex("实战项目");v1.edges = java.util.List.of(new Edge(v3)); // +1v2.edges = java.util.List.of(new Edge(v3)); // +1v3.edges = java.util.List.of(new Edge(v4));v6.edges = java.util.List.of(new Edge(v4));v4.edges = java.util.List.of(new Edge(v5));v5.edges = java.util.List.of(new Edge(v7));v7.edges = java.util.List.of();List<Vertex> graph = java.util.List.of(v1,v2,v3,v4,v5,v6,v7);for (Vertex vertex : graph) {for (Edge edge : vertex.edges) {edge.linked.inDegree += 1;}}List<Vertex> result = new ArrayList<>();Stack<Vertex> stack = new Stack<>();for (Vertex vertex : graph) {if(vertex.inDegree == 0 ){stack.push(vertex);}}while(!stack.isEmpty()){Vertex pop = stack.pop();result.add(pop);for (Edge edge : pop.edges) {edge.linked.inDegree--;if(edge.linked.inDegree == 0 && !edge.linked.visited){stack.add(edge.linked);edge.linked.visited = true;}}}if(result.size() != graph.size()){System.out.println("发现环");}for (Vertex vertex : result) {System.out.println(vertex);}}
7) 最短路径
Dijkstra
算法描述:
- 将所有顶点标记为未访问。创建一个未访问顶点的集合。
- 为每个顶点分配一个临时距离值
- 对于我们的初始顶点,将其设置为零
- 对于所有其他顶点,将其设置为无穷大。
- 每次选择最小临时距离的未访问顶点,作为新的当前顶点
- 对于当前顶点,遍历其所有未访问的邻居,并更新它们的临时距离为更小
- 例如,1->6 的距离是 14,而1->3->6 的距离是11。这时将距离更新为 11
- 否则,将保留上次距离值
- 当前顶点的邻居处理完成后,把它从未访问集合中删除
public class Dijkstra {public static void main(String[] args) {Vertex v1 = new Vertex("v1");Vertex v2 = new Vertex("v2");Vertex v3 = new Vertex("v3");Vertex v4 = new Vertex("v4");Vertex v5 = new Vertex("v5");Vertex v6 = new Vertex("v6");v1.edges = List.of(new Edge(v3, 9), new Edge(v2, 7), new Edge(v6, 14));v2.edges = List.of(new Edge(v4, 15));v3.edges = List.of(new Edge(v4, 11), new Edge(v6, 2));v4.edges = List.of(new Edge(v5, 6));v5.edges = List.of();v6.edges = List.of(new Edge(v5, 9));List<Vertex> graph = List.of(v1, v2, v3, v4, v5, v6);dijkstra(graph, v1);}private static void dijkstra(List<Vertex> graph, Vertex source) {ArrayList<Vertex> list = new ArrayList<>(graph);source.dist = 0;while (!list.isEmpty()) {// 3. 选取当前顶点Vertex curr = chooseMinDistVertex(list);// 4. 更新当前顶点邻居距离updateNeighboursDist(curr, list);// 5. 移除当前顶点list.remove(curr);}for (Vertex v : graph) {System.out.println(v.name + " " + v.dist);}}private static void updateNeighboursDist(Vertex curr, ArrayList<Vertex> list) {for (Edge edge : curr.edges) {Vertex n = edge.linked;if (list.contains(n)) {int dist = curr.dist + edge.weight;if (dist < n.dist) {n.dist = dist;}}}}private static Vertex chooseMinDistVertex(ArrayList<Vertex> list) {Vertex min = list.get(0);for (int i = 1; i < list.size(); i++) {if (list.get(i).dist < min.dist) {min = list.get(i);}}return min;}}
改进 - 优先级队列
- 创建一个优先级队列,放入所有顶点(队列大小会达到边的数量)
- 为每个顶点分配一个临时距离值
- 对于我们的初始顶点,将其设置为零
- 对于所有其他顶点,将其设置为无穷大。
- 每次选择最小临时距离的未访问顶点,作为新的当前顶点
- 对于当前顶点,遍历其所有未访问的邻居,并更新它们的临时距离为更小,若距离更新需加入队列
- 例如,1->6 的距离是 14,而1->3->6 的距离是11。这时将距离更新为 11
- 否则,将保留上次距离值
- 当前顶点的邻居处理完成后,把它从队列中删除
public class DijkstraPriorityQueue {public static void main(String[] args) {Vertex v1 = new Vertex("v1");Vertex v2 = new Vertex("v2");Vertex v3 = new Vertex("v3");Vertex v4 = new Vertex("v4");Vertex v5 = new Vertex("v5");Vertex v6 = new Vertex("v6");v1.edges = List.of(new Edge(v3, 9), new Edge(v2, 7), new Edge(v6, 14));v2.edges = List.of(new Edge(v4, 15));v3.edges = List.of(new Edge(v4, 11), new Edge(v6, 2));v4.edges = List.of(new Edge(v5, 6));v5.edges = List.of();v6.edges = List.of(new Edge(v5, 9));List<Vertex> graph = List.of(v1, v2, v3, v4, v5, v6);dijkstra(graph, v1);}private static void dijkstra(List<Vertex> graph, Vertex source) {PriorityQueue<Vertex> queue = new PriorityQueue<>(Comparator.comparingInt(v -> v.dist));source.dist = 0;for (Vertex v : graph) {queue.offer(v);}while (!queue.isEmpty()) {System.out.println(queue);// 3. 选取当前顶点Vertex curr = queue.poll();// 4. 更新当前顶点邻居距离if(!curr.visited) {updateNeighboursDist(curr, queue);curr.visited = true;}// 5. 移除当前顶点}for (Vertex v : graph) {System.out.println(v.name + " " + v.dist + " " + (v.prev != null ? v.prev.name : "null"));}}private static void updateNeighboursDist(Vertex curr, PriorityQueue<Vertex> queue) {for (Edge edge : curr.edges) {Vertex n = edge.linked;if (!n.visited) {int dist = curr.dist + edge.weight;if (dist < n.dist) {n.dist = dist;n.prev = curr;queue.remove(n); // 先删除再添加才能改变优先级queue.offer(n);}}}}}
问题
按照 Dijkstra 算法,得出
- v1 -> v2 最短距离2
- v1 -> v3 最短距离1
- v1 -> v4 最短距离2
事实应当是
- v1 -> v2 最短距离2
- v1 -> v3 最短距离0
- v1 -> v4 最短距离1
Bellman-Ford
public class BellmanFord {public static void main(String[] args) {// 正常情况/*Vertex v1 = new Vertex("v1");Vertex v2 = new Vertex("v2");Vertex v3 = new Vertex("v3");Vertex v4 = new Vertex("v4");Vertex v5 = new Vertex("v5");Vertex v6 = new Vertex("v6");v1.edges = List.of(new Edge(v3, 9), new Edge(v2, 7), new Edge(v6, 14));v2.edges = List.of(new Edge(v4, 15));v3.edges = List.of(new Edge(v4, 11), new Edge(v6, 2));v4.edges = List.of(new Edge(v5, 6));v5.edges = List.of();v6.edges = List.of(new Edge(v5, 9));List<Vertex> graph = List.of(v4, v5, v6, v1, v2, v3);*/// 负边情况/*Vertex v1 = new Vertex("v1");Vertex v2 = new Vertex("v2");Vertex v3 = new Vertex("v3");Vertex v4 = new Vertex("v4");v1.edges = List.of(new Edge(v2, 2), new Edge(v3, 1));v2.edges = List.of(new Edge(v3, -2));v3.edges = List.of(new Edge(v4, 1));v4.edges = List.of();List<Vertex> graph = List.of(v1, v2, v3, v4);*/// 负环情况Vertex v1 = new Vertex("v1");Vertex v2 = new Vertex("v2");Vertex v3 = new Vertex("v3");Vertex v4 = new Vertex("v4");v1.edges = List.of(new Edge(v2, 2));v2.edges = List.of(new Edge(v3, -4));v3.edges = List.of(new Edge(v4, 1), new Edge(v1, 1));v4.edges = List.of();List<Vertex> graph = List.of(v1, v2, v3, v4);bellmanFord(graph, v1);}private static void bellmanFord(List<Vertex> graph, Vertex source) {source.dist = 0;int size = graph.size();// 1. 进行 顶点个数 - 1 轮处理for (int i = 0; i < size - 1; i++) {// 2. 遍历所有的边for (Vertex s : graph) {for (Edge edge : s.edges) {// 3. 处理每一条边Vertex e = edge.linked;if (s.dist != Integer.MAX_VALUE && s.dist + edge.weight < e.dist) {e.dist = s.dist + edge.weight;e.prev = s;}}}}for (Vertex v : graph) {System.out.println(v + " " + (v.prev != null ? v.prev.name : "null"));}}
}
负环
如果在【顶点-1】轮处理完成后,还能继续找到更短距离,表示发现了负环
Floyd-Warshall
public class FloydWarshall {public static void main(String[] args) {Vertex v1 = new Vertex("v1");Vertex v2 = new Vertex("v2");Vertex v3 = new Vertex("v3");Vertex v4 = new Vertex("v4");v1.edges = List.of(new Edge(v3, -2));v2.edges = List.of(new Edge(v1, 4), new Edge(v3, 3));v3.edges = List.of(new Edge(v4, 2));v4.edges = List.of(new Edge(v2, -1));List<Vertex> graph = List.of(v1, v2, v3, v4);/*直接连通v1 v2 v3 v4v1 0 ∞ -2 ∞v2 4 0 3 ∞v3 ∞ ∞ 0 2v4 ∞ -1 ∞ 0k=0 借助v1到达其它顶点v1 v2 v3 v4v1 0 ∞ -2 ∞v2 4 0 2 ∞v3 ∞ ∞ 0 2v4 ∞ -1 ∞ 0k=1 借助v2到达其它顶点v1 v2 v3 v4v1 0 ∞ -2 ∞v2 4 0 2 ∞v3 ∞ ∞ 0 2v4 3 -1 1 0k=2 借助v3到达其它顶点v1 v2 v3 v4v1 0 ∞ -2 0v2 4 0 2 4v3 ∞ ∞ 0 2v4 3 -1 1 0k=3 借助v4到达其它顶点v1 v2 v3 v4v1 0 -1 -2 0v2 4 0 2 4v3 5 1 0 2v4 3 -1 1 0*/floydWarshall(graph);}private static void floydWarshall(List<Vertex> graph){int size = graph.size();int[][] dist = new int[size][size];for (int i = 0; i < size; i++) { // 初始化Vertex vertex = graph.get(i);Map<Vertex, Integer> collect = vertex.edges.stream().collect(Collectors.toMap(v -> v.linked, v -> v.weight));for (int i1 = 0; i1 < size; i1++) {if(i == i1){dist[i][i1] = 0;continue;}dist[i][i1] = collect.getOrDefault(graph.get(i1),Integer.MAX_VALUE);}}for (int k = 0; k < size; k++) {for (int j = 0; j < size; j++) {int dist1;if((dist1 = dist[j][k]) < Integer.MAX_VALUE){for (int i = 0; i < size; i++) {int dist2;if((dist2 = dist[k][i]) != Integer.MAX_VALUE)dist[j][i] = Integer.min(dist1 + dist2,dist[j][i]);}}}}for (int[] ints : dist) {for (int anInt : ints) {System.out.print(anInt + " ");}System.out.println();}}
}
负环
如果在 3 层循环结束后,在 dist 数组的对角线处(i==j 处)发现了负数,表示出现了负环
8) 最小生成树
图的最小生成树是一个子图,它是连通的,包含图中所有的顶点,并且所有边的权重之和最小。在最小生成树中,没有任何一条边可以被其他边替换而使得总权重变小。也就是说,最小生成树是图的所有生成树中,边的权值总和最小的生成树。
解决最小生成树问题的常用算法有Prim算法和Kruskal算法。Prim算法从一个顶点开始,每次都添加一条与当前子图连接的权重最小的边,直到所有顶点都被包含在子图中。Kruskal算法则是从所有的边开始,每次都添加一条当前所有边中权重最小的边,但需要保证添加的边不会形成环,直到所有顶点都被连接。
Prim
public class Prim {public static void main(String[] args) {Vertex v1 = new Vertex("v1");Vertex v2 = new Vertex("v2");Vertex v3 = new Vertex("v3");Vertex v4 = new Vertex("v4");Vertex v5 = new Vertex("v5");Vertex v6 = new Vertex("v6");Vertex v7 = new Vertex("v7");v1.edges = List.of(new Edge(v2, 2), new Edge(v3, 4), new Edge(v4, 1));v2.edges = List.of(new Edge(v1, 2), new Edge(v4, 3), new Edge(v5, 10));v3.edges = List.of(new Edge(v1, 4), new Edge(v4, 2), new Edge(v6, 5));v4.edges = List.of(new Edge(v1, 1), new Edge(v2, 3), new Edge(v3, 2),new Edge(v5, 7), new Edge(v6, 8), new Edge(v7, 4));v5.edges = List.of(new Edge(v2, 10), new Edge(v4, 7), new Edge(v7, 6));v6.edges = List.of(new Edge(v3, 5), new Edge(v4, 8), new Edge(v7, 1));v7.edges = List.of(new Edge(v4, 4), new Edge(v5, 6), new Edge(v6, 1));List<Vertex> graph = List.of(v1, v2, v3, v4, v5, v6, v7);prim(graph, v1);}static void prim(List<Vertex> graph, Vertex source) {ArrayList<Vertex> list = new ArrayList<>(graph);source.dist = 0;while (!list.isEmpty()) {Vertex min = chooseMinDistVertex(list);updateNeighboursDist(min);list.remove(min);min.visited = true;System.out.println("---------------");for (Vertex v : graph) {System.out.println(v);}}}private static void updateNeighboursDist(Vertex curr) {for (Edge edge : curr.edges) {Vertex n = edge.linked;if (!n.visited) {int dist = edge.weight;if (dist < n.dist) {n.dist = dist;n.prev = curr;}}}}private static Vertex chooseMinDistVertex(ArrayList<Vertex> list) {Vertex min = list.get(0);for (int i = 1; i < list.size(); i++) {if (list.get(i).dist < min.dist) {min = list.get(i);}}return min;}
}
Kruskal
private static void kruskal(List<Vertex> graph,Vertex v1){List<Edge> edges = new ArrayList<>();List<Vertex> pre = new ArrayList<>();for (Vertex vertex : graph) {for (Edge edge : vertex.edges) {edges.add(edge);pre.add(vertex);}}for (int i = 0; i < edges.size(); i++) {Edge minEdge = edges.get(i);int min = minEdge.weight;for (int j = i + 1 ; j < edges.size(); j++) {Edge e = edges.get(j);if(minEdge.weight > e.weight){edges.set(j,minEdge);minEdge = e;edges.set(i,e);Vertex v = pre.get(i);pre.set(i,pre.get(j));pre.set(j,v);}}}List<Vertex> used = new ArrayList<>();for (int i = 0; i < edges.size(); i++) {Vertex v = pre.get(i);Vertex e = edges.get(i).linked;if(!used.contains(v) || !used.contains(e)){System.out.println(v.name + " -> " + e.name);used.add(v);used.add(e);}}
}