# problem implementing Dijkstra's single source shortest path algorithm

I am trying to implement Dijkstra’s single source shortest path algorithm using priority queue data structure and I got stuck at some point.

I am using adjacency list representation of graph, I want to retrieve weight of edge(x,y) how to do it efficiently.

#include
#include
#include
#include
using namespace std;
#define INF INT_MAX //Infinity

``````    const int sz=10001; //Maximum possible number of vertices. Preallocating space for DataStructures accordingly
int dis[sz]; //Stores shortest distance
bool vis[sz]={0}; //Determines whether the node has been visited or not

void Dijkstra(int source, int n) //Algorithm for SSSP
{
for(int i=0;i<sz;i++) //Set initial distances to Infinity
dis[i]=INF;
//Custom Comparator for Determining priority for priority queue (shortest edge comes first)
class prioritize{public: bool operator ()(pair<int, int>&p1 ,pair<int, int>&p2){return p1.second>p2.second;}};
priority_queue<pair<int,int> ,vector<pair<int,int> >, prioritize> pq; //Priority queue to store vertex,weight pairs
pq.push(make_pair(source,dis[source]=0)); //Pushing the source with distance from itself as 0
while(!pq.empty())
{
pair<int, int> curr=pq.top(); //Current vertex. The shortest distance for this has been found
pq.pop();
int cv=curr.first,cw=curr.second; //'cw' the final shortest distance for this vertex
if(vis[cv]) //If the vertex is already visited, no point in exploring adjacent vertices
continue;
vis[cv]=true;
for(int i=0;i<a[cv].size();i++) //Iterating through all adjacent vertices
if(!vis[a[cv][i].first] && a[cv][i].second+cw<dis[a[cv][i].first]) //If this node is not visited and the current parent node distance+distance from there to this node is shorted than the initial distace set to this node, update it
pq.push(make_pair(a[cv][i].first,(dis[a[cv][i].first]=a[cv][i].second+cw))); //Set the new distance and add to priority queue
}
}

int main() //Driver Function for Dijkstra SSSP
{
int n,m,x,y,w;//Number of vertices and edges
//cout<<"Enter number of vertices and edges in the graph\n";
cin>>n>>m;
for(int i=0;i<m;i++) //Building Graph
{
cin>>x>>y>>w; //Vertex1, Vertex2, weight of edge
a[x].push_back(make_pair(y,w));
a[y].push_back(make_pair(x,w));
}
//cout<<"Enter source for Dijkstra's SSSP algorithm\n";
int source;
cin>>source;
Dijkstra(source,n);//SSSP from source (Also passing number of vertices as parameter)
cout<<"Source is: "<<source<<". The shortest distance to every other vertex from here is: \n";
for(int i=1;i<=n;i++)//Printing final shortest distances from source
{
cout<<"Vertex: "<<i<<" , Distance: ";
dis[i]!=INF? cout<<dis[i]<<"\n" : cout<<"-1\n";
}
return 0;
}
``````

Here is the code in action . If you have any queries ask in the comments !

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@sarvagya3943

I am coding in java, I am not familiar with STL so I am not getting this code.

I tried explaining the code here with usage of STL specifically. Take a look .

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@sarvagya3943

why priority queue is declared like this
priority_queue<pair<int,int> ,vector<pair<int,int> >pq;

why does it have vector in it.

There are three parameters required here .

1. type of the elements (`pair<int,int>`)

2. Type of the internal underlying container object where the elements are stored.(that’s what vector is used here for )

3. A binary predicate that takes two elements (of type T) as arguments and returns a bool.(this is optional, by default - max priority_queue )

Reference

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