有向图中的前后边缘

Question

I am new to graph theory. I am writing a code to find all forward and back edges in directed graph. I searched and implemented the code as below. My code is going into infinite loop. Could you please have a look. We will very greateful to you.

void dfsVisit(u)
      {
        u->color="gray";
        u->time=count;
        count++;
        for(every child node v of u)
        {
           if(v->color == "black")
           {
              if(u->time < v->time)
                  cout<<"Edge "<<u<<"->"<<v<<" is forward edge"<<endl;
              else
                  cout<<"Edge "<<u<<"->"<<v<<" is cross edge"<<endl;
            }
            if(v->color == "gray")
                 cout<<"Edge "<<u<<"->"<<v<<" is back edge"<<endl;
           if(v->color == "white")
                 cout<<"Edge "<<u<<"->"<<v<<" is tree edge"<<endl;
          dfsVisit(v); 
         }
         u->color="black";
         u->time=count;
         count++;
    }

Answer 1

I see two possible problems with your code:

1. you call dfsVisit for every child, regardless of its color. You indented it into a deeper level, but you didn't add any parenthesis around the white if - based on this pseudocode, only the cout is affected by the if statement. But since this is just a pseudocode, you should post the original C++ code you wrote to be clear.

2. you have three colors in your conditions (grey, black, and white), but you never set white anywhere. Your edges are initialized to white before you call dfsVisit ? (you should consider using an enum or enum class for the colors, it's cleanear and you'll less likely to cause bugs with typos)