Dijkstra算法Java实现

标签：最短路径   dijkstar   贪心算法   算法   java   

从最短路径开始，学习Dijstar算法

下面是求最短路径问题描述：

下面是Dijkstar算法的描述：

下面是使用Dijkstar算法求解右图中的问题：

下面是图的邻接矩阵：

下面是计算过程：

下面是求解最短路径：

下面是Java代码实现，其中需要注意的问题我已经标出（一定要细心！！！！）:

package greedy_algorithm;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;

public class Dijkstra {

	private Set<Integer> permanent=new HashSet<>();
	private Set<Integer> temporary=new HashSet<>();
	private Map<Integer, Integer> nodeMap=new HashMap<Integer, Integer>(); //key=永久节点，value=最短路径前一个节点,提高回溯效率
	private int [][] m;
	private Mark [][] d;
	private int n;
	private final static int MAX=65535;
	
	class Mark{
		private int length=MAX;
		private int previous=-1;
		private boolean isMarked=false;
		
		public Mark(){}
		public Mark(int length,int previous,boolean isMarked){
			this.length=length;
			this.previous=previous;
			this.isMarked=isMarked;
		}
	}
	
	public Dijkstra(){
		n=6;
		m=new int[][]{{ 0, 1, 4,-1,-1,-1},
				      { 1, 0, 2, 7, 5,-1},
				      { 4, 2, 0,-1, 1,-1},
				      {-1, 7,-1, 0, 3, 2},
				      {-1, 5, 1, 3, 0, 6},
				      {-1,-1,-1, 2, 6, 0}};
		d=new Mark[n][n];
		
		for(int i=0;i<n;i++){
			for(int j=0;j<n;j++){
				d[i][j]=new Mark();
				//System.out.println(d[i][j].length+"  "+d[i][j].previous+"  "+d[i][j].isMarked);
			}
		}
	}
	
	public Dijkstra(int n,int [][]m,Mark [][]d){
		this.n=n;this.m=m;this.d=d;
		if(n!=m.length && n!=m[0].length &&
		   n!=d.length && n!=d[0].length){
			System.out.println("输入格式有误");
			System.exit(1);
		}
	}
	
	public void shortestPath(){
		ArrayList<Mark> vList;
		permanent.add(0);
		for(int i=1;i<n;i++){
			temporary.add(i);
		}
		d[0][0]=new Mark(0,-1,true);
		nodeMap.put(0, -1);
		/*for(int q=0;q<n;q++)
			System.out.print("("+d[0][q].length+","+d[0][q].previous+","+d[0][q].isMarked+")  ");
		System.out.println();*/
		for(int i=1;i<n;i++){
			for(int t:temporary){
				vList=new ArrayList<Dijkstra.Mark>();
				for(int p:permanent){
					if(m[t][p]!=-1){
						vList.add(new Mark(getLength(t, p),p,false));
					}
				}
				if(vList.size()!=0){
					d[i][t]=getMin(vList);
				}
			}
			markNode(i);
			/*for(int r=0;r<n;r++)
				System.out.print("("+d[i][r].length+","+d[i][r].previous+","+d[i][r].isMarked+")  ");
			System.out.println();*/
		}
		
	}
	
	public void traceBack(int end){
		int node=end;
		for(int i=0;i<n;i++){
			if(d[i][node].isMarked==true){
				System.out.println("The shortest lenth from V0 to V"+node+" is:"+d[i][node].length);
			}
		}
		System.out.println("The shortest path is:");
		trace(end);
	}
	
	public void trace(int end){
		int node =end;
		if(node>=0){
			trace(nodeMap.get(node));
			if(node==5)
				System.out.print("V"+node);
			else
				System.out.print("V"+node+"->");
		}
	}
	
	public int getLength(int t,int p){
		int length=m[t][p];
		for(int i=0;i<n;i++){
			if(d[i][p].isMarked==true){
				length+=d[i][p].length;
				break;
			}
		}
		return length;
	}
	
	public Mark getMin(ArrayList<Mark> vList){
		Mark mark=new Mark();
		int min=MAX;            //注意这里一定是min=MAX，这个地方的问题查了好久的错误！！！
		//System.out.println("min="+min);
		for(Mark c:vList){
			//System.out.println("("+c.length+","+c.previous+","+c.isMarked+")");
			if(min>c.length){
				min=c.length;
				mark=c;
				//System.out.println("break");
			}
		}
		//System.out.println("("+mark.length+","+mark.previous+","+mark.isMarked+")");
		return mark==null?new Mark(-1,-1,false):mark;
	}
	
	public void markNode(int row){
		int min=MAX;
		int position=-1;
		for(int j=0;j<n;j++){
			if(d[row][j].length!=MAX){
				if(min>d[row][j].length){
					min=d[row][j].length;
					position=j;
				}
			}
		}
		if(position!=-1){
			d[row][position].isMarked=true;
			permanent.add(position);
			temporary.remove(position);
			nodeMap.put(position, d[row][position].previous);
		}
	}
	public static void main(String[] args) {
		Dijkstra dijkstra=new Dijkstra();
		dijkstra.shortestPath();
		/*for(Map.Entry<Integer, Integer> entry:dijkstra.nodeMap.entrySet())
			System.out.println(entry);*/
		dijkstra.traceBack(5);
	}
}

Dijkstra算法Java实现

标签：最短路径   dijkstar   贪心算法   算法   java   

原文地址：http://blog.csdn.net/weiweiyixiaocsdn/article/details/45815347