迪杰斯特拉

找到某一个顶点到其他所有顶点的最短距离

弗洛伊德

找到所有顶点之间的最短距离

package Algorithm.floyd;

import java.util.Arrays;

public class floydDemo {

    public static void main(String[] args) {
        final int N = 65535;
        char[] vertex = {'A','B','C','D','E','F','G'};
        int[][] weight = new int[][] {
            {0,5,7,N,N,N,2},
            {5,0,N,9,N,N,3},
            {7,N,0,N,8,N,N},
            {N,9,N,0,N,4,N},
            {N,N,8,N,0,5,4},
            {N,N,N,4,5,0,6},
            {2,3,N,N,4,6,0},
            };
        Graph graph = new Graph(vertex.length, weight, vertex);
        graph.floyd();
        graph.showAll();
    }
}

class Graph{
    private char[] vertex;
    private int[][] dis;
    private int[][] pre;
    
    public Graph(int length, int[][] dis, char[] vertex) {
        this.vertex = vertex;
        this.dis = dis;
        this.pre = new int[length][length];
        for(int i=0; i<vertex.length; i++) {
            Arrays.fill(pre[i], i);   //pre[i]是第i行
        }
    }
    
    public void showAll() {
        
        for(int i=0; i<vertex.length; i++) {
            //输出pre
            for(int j=0; j<vertex.length; j++) {
                System.out.print(pre[i][j] + " ");
            }
            System.out.println();
            //输出dis
            for(int j=0; j<vertex.length; j++) {
                System.out.print(dis[i][j] + " ");
            }
            System.out.println();
        }
    }
    
    public void floyd() {
        int len;   // 临时变量记录经过中间节点的新的距离
        for(int k=0; k<vertex.length; k++) {   // k表示中间节点
            for(int i=0; i<vertex.length; i++) {  // i表示出发节点
                for(int j=0; j<vertex.length; j++) {
                    len = dis[i][k] + dis[k][j];
                    if(len < dis[i][j]) {   //如果len小于两点之间的直接距离，就可以替换了
                        dis[i][j] = len;    // update the distance between i and j
                        pre[i][j] = pre[k][j];   // update the pre-vertex of i,j
                    }
                }
            }
        }
    }
    
}