/// <summary>
/// 用edges[]中的边来构造一幅加权有向图来检测环——基本思路：
/// 在图中dfs，判断到目前节点已被marked并且在栈中，就是一个环
/// </summary>
private void findNegativeCycle()
{
int V = edges.Length;
DirectedWeightedEdgeGraph G = new DirectedWeightedEdgeGraph(V);
foreach (var e in edges)//用edges来构造一幅加权有向图
if (e != null)
G.AddEdge(e);
DirectedWeightedCycle dc = new DirectedWeightedCycle(G);
cycle = dc.GetCycle();//疑问：貌似找到的不一定是负权重环？？？？
}
private void relax(int v, DirectedWeightedEdgeGraph G)
{
foreach (var e in G.GetAdj(v))
{
int w = e.To();
if (disTo[w] > disTo[v] + e.GetWeight())
{
disTo[w] = disTo[v] + e.GetWeight();
edges[w] = e;
if (!inQueue[w])
{
inQueue[w] = true;
queue.Enqueue(w);
}
}
if (cost++ % G.GetV() == 0)
findNegativeCycle();
}
}
public BellmanFord(DirectedWeightedEdgeGraph G, int s)
{
disTo = new double[G.GetV()];
edges = new DirectedWeightedEdge[G.GetE()];
inQueue = new bool[G.GetV()];
queue = new Queue<int>();
for (int i = 0; i < G.GetV(); i++)
disTo[i] = double.MaxValue;
disTo[s] = 0;
inQueue[s] = true;
queue.Enqueue(s);
while (queue.Count() > 0 && !hasNegativeCycle())//没有负权重环出现
{
int v = queue.Dequeue();
inQueue[v] = false;
relax(v, G);
}
}
private void dfs(int v, DirectedWeightedEdgeGraph G)
{
marked[v] = true;
onstack[v] = true;
foreach (var e in G.GetAdj(v))
{
var w = e.To();
if (cycle != null)
{
return;
}
else if (!marked[w])
{
edgeTo[w] = e;
dfs(w, G);
}
else if (onstack[w])
{
cycle = new Stack<DirectedWeightedEdge>();
for (var ee = e; ee.From() != w; ee = edgeTo[e.From()])
cycle.Push(e);
cycle.Push(e);
}
}
onstack[v] = false;
}
public DirectedWeightedCycle(DirectedWeightedEdgeGraph G)
{
marked = new bool[G.GetV()];
onstack = new bool[G.GetV()];
edgeTo = new DirectedWeightedEdge[G.GetE()];
for (int v = 0; v < G.GetV(); v++)
if (!marked[v]) dfs(v, G);
}
