最短路径：迪杰斯特拉算法

简介

        英文名Dijkstra

        作用：找到路中指定起点到指定终点的带权最短路径

核心步骤

        1）确定起点，终点

        2）从未走过的点中选取从起点到权值最小点作为中心点

        3）如果满足 起点到中心点权值 + 中心点到指定其他点的权值 < 起点到其他点的权值，

        即Weight[start] [center] +Weight [center] [other] < Weight [start] [center] ,

        简言之，有更短的路径就取更短的路

理论模拟

        以A为起点，D为终点，如图所示 径, 更新记录更短权值路径

 从未走过的点中选取权值最小点，即A作为中心点，标记A走过，更新起点到B、F、G的路径

 

从未走过的点中选取权值最小点，即B, 并且W:B->C + W:A->C = 12 + 10 < +oo ，更新起点A到C的路径和,

即W: A-> C =W: A-> B -> C =12+10 =22

 

 继续从未走过的点中选取权值最小点G, W: A -> E =+oo > W: A->G ->E =14+8 =22 ,

 更新W: A->E 为22

选取F, 由于W:A->F->E=16+2 =18 <22 更新 W: A-> E =18 ,

 选取E,由于W:A->E->D=18+4=22<+oo,则更新W: A->D =22

 选取C,无可更新点

 到达终点D! 最短路径为A->F->E->D ,最短路径和为22

 

Java代码实现

 顶点

//顶点类
public class Vertex {public String Number;  //顶点编号public List<Vertex>neighborVertexs;    //邻居顶点public Map<Vertex,Integer>weights;     //与邻居节点之间的权值public Vertex(String number) {this.Number = number;this.neighborVertexs=new LinkedList<>();this.weights=new HashMap<>();}
}

边

public class Edge {public Vertex start;public Vertex end;public Integer weight;public Edge(Vertex start, Vertex end, Integer weight) {this.start = start;this.end = end;this.weight = weight;}
}

最短路径返回结果

public class MinPathResult {public String minPathString;    //将最短路径拼接成字符串形式，如 A->B->Cpublic List<Vertex>minPathList; //将起点到终点的路径储存在List集合中public Integer minPathSum;  //记录起点到终点的最短路径和public MinPathResult(List<Vertex> minPathList, String minPathString,Integer minPathSum) {this.minPathString = minPathString;this.minPathList = minPathList;this.minPathSum=minPathSum;}@Overridepublic String toString() {return "Result{" +"minPathString:'" + minPathString +"  minPathSum:"+minPathSum+'}';}
}

Dijkstra算法的实现,适用于无向图

import java.util.*;
//适用于无向图
public class DijkstraOperator {private List<Vertex>vertexs;    //全部顶点private List<Edge>edges;        //所有边private Map<String,Vertex>vertexs_map;  //通过顶点编号找到顶点private final static Integer INF=Integer.MAX_VALUE;     //代表无穷大public DijkstraOperator(List<Vertex> vertexs, List<Edge> edges) {this.vertexs = vertexs;this.edges = edges;this.vertexs_map=new HashMap<>();//构建编号映射顶点for(Vertex v:vertexs){vertexs_map.put(v.Number,v);}//填充所有顶点的邻居以及权值for(int i=0;i<edges.size();i++){//填充起点的邻居，以及起点到终点的权值edges.get(i).start.neighborVertexs.add(edges.get(i).end);edges.get(i).start.weights.put(edges.get(i).end,edges.get(i).weight);//填充终点的邻居，以及终点到起点的权值edges.get(i).end.neighborVertexs.add(edges.get(i).start);edges.get(i).end.weights.put(edges.get(i).start,edges.get(i).weight);}}//获得从起点到终点之间的路径public MinPathResult getMinPath(String start, String end){//用哈希表标记某个顶点是否走过Map<Vertex,Boolean>visited=new HashMap<>();//用哈希表记录顶点的前驱Map<Vertex,Vertex>preVertex=new HashMap<>();//利用哈希表记录起点到任意一点的最短路径Map<Vertex,Integer>minPath=new HashMap<>();//初始化三个表for(int i=0;i<vertexs.size();i++){//初始化每一个点都未走过visited.put(vertexs.get(i),false);//初始化每个点的前驱都是自己preVertex.put(vertexs.get(i),vertexs.get(i));//初始化起点到任意两个点之间的最短路径都是无穷大minPath.put(vertexs.get(i),INF);}Vertex startVertex=vertexs_map.get(start);Vertex endVertex=vertexs_map.get(end);//填充存在的路径for(int i=0;i<startVertex.neighborVertexs.size();i++){//设置起点与邻居节点之间的权值minPath.put(startVertex.neighborVertexs.get(i),startVertex.weights.get(startVertex.neighborVertexs.get(i)));//设置点前驱preVertex.put(startVertex.neighborVertexs.get(i),startVertex);}//自己到自己的距离为0minPath.put(startVertex,0);Vertex curVertex=null;//一直寻路，直到找到终点while(curVertex!=endVertex){Integer minWeight=Integer.MAX_VALUE;curVertex=null;//能看到的点之间选取距离最小的那个,且这个点并没有走过for(Vertex v:minPath.keySet()){if(!visited.get(v)&&minPath.get(v)<minWeight){//切换中心点curVertex=v;//更新最小权值minWeight=minPath.get(v);}}//如果找不到下一个中心点，说明从起点根本到达不来终点if(curVertex==null)return null;//标记选取点visited.put(curVertex,true);//更新权值for(int i=0;i<curVertex.neighborVertexs.size();i++){//邻居节点Vertex neighborVertex=curVertex.neighborVertexs.get(i);//计算起点到邻居节点之间新的权值int newWeight=minPath.get(curVertex)+curVertex.weights.get(neighborVertex);//找到能移动的点,如果转折之后距离更短，则记录更短的距离if(visited.get(neighborVertex)==false&&newWeight<minPath.get(neighborVertex)){//记录更短距离minPath.put(neighborVertex,newWeight);//记录邻居节点的前驱preVertex.put(neighborVertex,curVertex);}}}//起点到终点之间的最短路径LinkedList<Vertex>targetPath=new LinkedList<>();for(Vertex curVer=endVertex;curVer!=startVertex;curVer=preVertex.get(curVer)){targetPath.addFirst(curVer);}targetPath.addFirst(startVertex);//拼接最短路径StringBuffer minPathStringBuffer=new StringBuffer();Integer pathSum=0;for(int i=0;i< targetPath.size();i++){minPathStringBuffer.append(targetPath.get(i).Number);if(i!= targetPath.size()-1){pathSum=pathSum+targetPath.get(i).weights.get(targetPath.get(i+1));minPathStringBuffer.append("->");}}return new MinPathResult(targetPath, minPathStringBuffer.toString(),pathSum);}
}

测试函数

import java.util.*;public class Main {public static void main(String[] args) {Scanner scanner=new Scanner(System.in);List<Vertex>vertexs=new LinkedList<>();List<Edge>edges=new LinkedList<>();System.out.println("请输入顶点的数量:");Integer vexcnt= scanner.nextInt();System.out.println("请输入这些顶点编号:");for(int i=0;i<vexcnt;i++){vertexs.add(new Vertex(scanner.next()));}System.out.println("请输入边的数量：");Integer edgecnt= scanner.nextInt();System.out.println("请输入这些边的端点编号和权值:");for(int i=0;i<edgecnt;i++){String number1= scanner.next();String number2= scanner.next();Integer weight= scanner.nextInt();Vertex v1=null;Vertex v2=null;for(int j=0;j<vertexs.size();j++){if(vertexs.get(j).Number.equals(number1))v1=vertexs.get(j);if(vertexs.get(j).Number.equals(number2))v2=vertexs.get(j);}edges.add(new Edge(v1,v2,weight));}//定义迪杰斯特拉操作类DijkstraOperator dijkstra=new DijkstraOperator(vertexs,edges);//获取任意两点之间的最短路径结果集List<MinPathResult>minPathResults=new ArrayList<>();for(int i=0;i< vertexs.size();i++){for(int j=i+1;j< vertexs.size();j++){minPathResults.add(dijkstra.getMinPath(vertexs.get(i).Number,vertexs.get(j).Number));System.out.println(minPathResults.get(minPathResults.size()-1));}}}
}

测试输入与输出结果

//输入部分
请输入顶点的数量:
7
请输入这些顶点编号:
A B C D E F G
请输入边的数量：
12
请输入这些边的端点编号和权值:
A B 12
A F 16
A G 14
B C 10
B F 7
G F 9
G E 8
F C 6
F E 2
C D 3
C E 5
E D 4//输出部分
Result{minPathString:'A->B  minPathSum:12}
Result{minPathString:'A->B->C  minPathSum:22}
Result{minPathString:'A->F->E->D  minPathSum:22}
Result{minPathString:'A->F->E  minPathSum:18}
Result{minPathString:'A->F  minPathSum:16}
Result{minPathString:'A->G  minPathSum:14}
Result{minPathString:'B->C  minPathSum:10}
Result{minPathString:'B->F->E->D  minPathSum:13}
Result{minPathString:'B->F->E  minPathSum:9}
Result{minPathString:'B->F  minPathSum:7}
Result{minPathString:'B->F->G  minPathSum:16}
Result{minPathString:'C->D  minPathSum:3}
Result{minPathString:'C->E  minPathSum:5}
Result{minPathString:'C->F  minPathSum:6}
Result{minPathString:'C->E->G  minPathSum:13}
Result{minPathString:'D->E  minPathSum:4}
Result{minPathString:'D->E->F  minPathSum:6}
Result{minPathString:'D->E->G  minPathSum:12}
Result{minPathString:'E->F  minPathSum:2}
Result{minPathString:'E->G  minPathSum:8}
Result{minPathString:'F->G  minPathSum:9}进程已结束，退出代码为 0