单源最短路径 : Bellman-Ford 算法

public class BellmanFord {

static boolean BellmanFord(int[][] graph, char s) {
int[] p = new int[graph.length];
int[] dist = new int[graph.length];

for(int i = 0; i <  graph.length; i++) {
dist[i] = M;
p[i] = -1;
}

int start = s - 'A';
dist[start] = 0;
p[start] = -1;

//max n-1 edges
for(int n = 1; n <  graph.length; n++) {
for(int i = 0; i < graph.length; i++) {
for(int j = 0; j < graph[i].length; j++) {
if(dist[j] > (dist[i] + graph[i][j])) { //relaxation
dist[j] = (dist[i] + graph[i][j]);
p[j] = i;
}
}
}
}

boolean flag = true;
//检查是否含有负权环路
for(int i = 0; i < graph.length; i++) {
for(int j = 0; j < graph[i].length; j++) {
if(dist[j] > (dist[i] + graph[i][j])) {
flag = false;
break;
}
}
}

//print paths
if(flag) {
for(int i = 0; i < graph.length; i++) {

int j = i;
if (dist[j] < M) {
System.out.printf("%c -> %c (%d) : ", s, (char)('A' + i), dist[i]);
StringBuilder sb = new StringBuilder();
sb.append((char)('A' + j));
while (p[j] != -1) {
sb.insert(0, (char) ('A' + p[j]) + " - ");
j = p[j];
}
System.out.println(sb.toString());
} else {
System.out.printf("%c -> %c (∞) : \n", s, (char)('A' + i));
}
}
}

return flag;
}

static final int M = 10000; //unreachable
public static void main(String[] args) {
int[][] graph = {
{0, 6, 7, M, M},
{M, 0, 8, 5, -4},
{M, M, 0, -3, 9},
{M, -2, M, 0, M},
{2, M, M, 7, 0}
};

BellmanFord(graph, 'A');
}
}