C ++：BFS计算树中每对节点之间的距离

Question

I have a weighted tree with N vertices stored in form of adjacency list. I have a list of M nodes.

Now to calculate distance between every pair of nodes from the list of M nodes in this tree I wrote this :

using namespace std;
#define MAX_N (1<<17)
#define MAX_V (1<<17)

typedef pair<int,int> pii;    
vector<pii> adj[MAX_V];

bool vis[MAX_N]; //mark the node if visited 
ll bfs(pii s,pii d)
{
    queue <pii> q;
    q.push(s);
    ll dist=0;

    vis[ s.first ] = true;
    while(!q.empty())
    {
        pii p = q.front();
        q.pop();
        for(auto i = 0 ; i < adj[ p ].size() ; i++)
        {
            if(vis[ adj[ p ][ i ].first ] == false)
            {
                 q.push(adj[ p ][ i ].first);
                 vis[ adj[ p ][ i ] ] = true;
                 dist += adj[p][i].second;
            }


        }
    }

    return dist;
}

 int main()
 {

        for(int i=0;i<N;i++)
        {
            int v1,v2,l;
            cin>>v1>>v2>>l;
            adj[v1].push_back(make_pair(v2,l));
           //  adj[v2].push_back(make_pair(v1,l));
        }
        int a[M];
        for(int i=0;i<M;i++)
        cin >> a[i];

        int ans=0;


      for(int i=0;i<M-1;i++)
        {
            for(int j=i+1;j<M;j++)
            {
                num += bfs(adj[a[i]],adj[a[j]]);
            }
        }

  }

The program doesn't compile and the error is as follows :

could not convert 'adj[a[i]]' from 'std::vector<std::pair<long long int, long long int> >' to 'pii {aka std::pair<long long int, long long int>}'
              num += bfs(adj[a[i]],adj[a[j]]);

Also I know this program is wrong while calculating BFS because it isn't stopping when it reaches the destination vertex.

Can someone help me out in correcting these errors ?

Answer 1

I think there are several issues:

• for(auto i = 0 ; i < adj[ p ].size() ; i++) you are using p as index for adj, but p is of type pii . You probably want p.first, assuming that the pairs have meaning (from, to)
• if(vis[ adj[ p ][ i ].first ] == false) : I assume you want to do check if the neighbours are not visited yet. So it should more something like this: vis[adj[p.first][i].second] == false . The index for visited is adj[p.first][i].second . I am checking second because I assume the semantic for (from, to) for the pair
• q.push(adj[ p ][ i ].first); : you are pushing an integer, but the queue holds type pii . Up to you how you want to change it.
• dist += adj[p][i].second; : you are indexing the array using the pair. You should use an index
• Finally num += bfs(adj[a[i]],adj[a[j]]); as Buckster already explained in the comment you are passing vector<pii> instead of pii to the the bfs function

These are only compile issues though. I am not sure your algorithms actually does what you expect. You can use bfs to compute the distance between any couple of node, but if it is weighted, bfs per se does not give you the minimum path. If you are interested in the minimum path, you can use Dijkstra, provided that weights are positive. Here I have a bfs implementation that you can check, if you want, but it slightly more complex of what you are trying to do here.