SPFA

求最短路：

import java.util.LinkedList;
import java.util.Queue;
import java.util.Scanner;

public class Main {

    private static final int INF = Integer.MAX_VALUE;

    public static void main(String[] args) {

        Scanner in = new Scanner(System.in);

        while (in.hasNext()) {

            int n = in.nextInt();
            int m = in.nextInt();

            int[] dist = new int[n + 1];
            for (int i = 1; i <= n; ++ i) {
                dist[i] = INF;
            }
            dist[1] = 0;

            int edgeIndex = 0;
            int[] lastEdge = new int[n + 1];
            int[] end = new int[m + 1];
            int[] weight = new int[m + 1];
            int[] previous = new int[m + 1];

            while (m -- > 0) {
                int a = in.nextInt();
                int b = in.nextInt();
                int c = in.nextInt();
                edgeIndex ++;
                end[edgeIndex] = b;
                weight[edgeIndex] = c;
                previous[edgeIndex] = lastEdge[a];
                lastEdge[a] = edgeIndex;
            }

            Queue<Integer> queue = new LinkedList<>();
            boolean[] visited = new boolean[n + 1];
            visited[1] = true;
            queue.add(1);

            while (! queue.isEmpty()) {
                Integer top = queue.poll();

                visited[top] = false;

                for (int i = lastEdge[top]; i != 0; i = previous[i]) {

                    if (dist[end[i]] > dist[top] + weight[i]) {
                        dist[end[i]] = dist[top] + weight[i];
                        if (! visited[end[i]]) {
                            queue.offer(end[i]);
                            visited[end[i]] = true;
                        }
                    }

                }
            }

            System.out.println(dist[n] == INF ? "impossible" : dist[n]);

        }
    }
}

判斷是否有環：

import java.util.LinkedList;
import java.util.Queue;
import java.util.Scanner;

public class Main {

    private static final int INF = Integer.MAX_VALUE;

    public static void main(String[] args) {

        Scanner in = new Scanner(System.in);

        while (in.hasNext()) {

            int n = in.nextInt();
            int m = in.nextInt();

            int[] dist = new int[n + 1];
            // 能夠不初始化

            int edgeIndex = 0;
            int[] cnt = new int[n + 1];
            int[] lastEdge = new int[n + 1];
            int[] end = new int[m + 1];
            int[] weight = new int[m + 1];
            int[] previous = new int[m + 1];

            while (m -- > 0) {
                int a = in.nextInt();
                int b = in.nextInt();
                int c = in.nextInt();
                edgeIndex ++;
                end[edgeIndex] = b;
                weight[edgeIndex] = c;
                previous[edgeIndex] = lastEdge[a];
                lastEdge[a] = edgeIndex;
            }

            Queue<Integer> queue = new LinkedList<>();
            boolean[] visited = new boolean[n + 1];

            // 全部點出發是否有負環
            for (int i = 1; i <= n; ++ i) {
                visited[i] = true;
                queue.add(i);
            }

            boolean exist = false;

            while (! queue.isEmpty() && ! exist) {
                Integer top = queue.poll();

                visited[top] = false;

                for (int i = lastEdge[top]; i != 0; i = previous[i]) {

                    if (dist[end[i]] > dist[top] + weight[i]) {
                        dist[end[i]] = dist[top] + weight[i];
                        cnt[end[i]] ++;
                        if (cnt[end[i]] >= n) {
                            exist = true;
                            break;
                        }
                        if (! visited[end[i]]) {
                            queue.offer(end[i]);
                            visited[end[i]] = true;
                        }
                    }
                }
            }

            System.out.println(exist);

        }
    }
}