go_rob's blog

By go_rob, history, 8 years ago, In English

There are n nodes(1,2.. n) in a weighted graph with m (m>n) bi-directional edges.

All weights are positive and there are no multiple edges.

There is one fixed origin(Node 1). We will always start traveling from the origin.

Now we have to find the number of ways to reach each of the nodes(starting from the origin) with minimum cost.(All paths counted should have the minimum cost to reach that node).

Expected complexity O(m*log(m)).

Input — First line contains n , m .

Next m lines contain u,v,w. => There is an edge between u and v of weight w(w>0).

Output — Print n-1 lines one for each node other than origin, containing the number of ways to reach each of the nodes(starting from the origin) with minimum cost.

Sample Test -

4 5

1 2 5

1 3 3

3 2 2

3 4 7

2 4 5

Output —

2

1

3

Here is my solution, Please check if this will work for all cases, I did modified Dijkstra + DP.

#include <bits/stdc++.h>
using namespace std;
 
#define N 5001
#define fi first
#define se second
#define MOD 1000000007
 
#define pb push_back
 
#define ll long long
 
#define eps 1.0e-9
#define inf 1e9 + 5
#define double long double
 
#define pp pair<int,int>

vector< pp > adj[N];
int dis[N],dp[N];

int main()
{
  ios::sync_with_stdio(0);
  int i,j,k,m,n,t;
  cin>>n>>m;
  for(i=0;i<m;i++)
  {
    int u,v,w;
    cin>>u>>v>>w;
    //u--;v--;
    adj[u].pb({w,v});
    adj[v].pb({w,v});
  }
  priority_queue< pp , vector<pp > , greater<pp> > pq;
  for(int i=0;i<=n;i++) dis[i] = INT_MAX;
  dis[1] = 0;
  dp[1] = 1;
  pq.push({0,1});

  while(!pq.empty())
  {
    pp tp = pq.top();
    int u = tp.se;
    int d = tp.fi;
    pq.pop();
    if(dis[u]<d) continue;

    for(i=0;i<adj[u].size();i++)
    {
      int v = adj[u][i].se;
      int w = adj[u][i].fi;
      if(d + w <= dis[v] )
      {
        if(dis[v]==d+w) dp[v] += dp[u];

        if(d + w < dis[v])
        {
          dis[v] = d + w;  
          dp[v] = dp[u];
          pq.push({dis[v],v}); 
        }
      }
    }
  }
  
  cout<<endl;

  for(int i = 1;i<=n;i++)
  {
    cout<<i<<" dis = "<<dis[i]<<" ways = "<<dp[i]<<endl;
  }
return 0;
}
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| Write comment?
»
8 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Auto comment: topic has been updated by go_rob (previous revision, new revision, compare).

»
8 years ago, # |
  Vote: I like it 0 Vote: I do not like it

I think your idea is correct, but I found a small bug.
Your solution fails for this testcase:
3 3
1 2 1
1 3 3
2 3 1

The reason is that you are not resetting dp[v] when (dis[v] > d + w)

»
8 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Guys I have updated my code. Please review this code and ignore the above code. Sorry for the inconvenience.

»
8 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Auto comment: topic has been updated by go_rob (previous revision, new revision, compare).

»
8 years ago, # |
  Vote: I like it -16 Vote: I do not like it

meaningless defines, unnecessary global variables, pure cancer called bits/stdc++.h. Do not show this shit to your future employer

  • »
    »
    8 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Why bits/stdc++.h> considered to be shit?