Error in Dijkstra's shortest path algorithm implementation (using C++ STL)

Question

I have tried to implement Dijkstra's algorithm in C++ using std::priority_queue.The function "dijsktra" will take input as 2 nodes, the source vertex and the destination vertex. However, I am getting incorrect answers every time. Kindly help me out and tell me where I have erred. My code-

#include <iostream>
#include <cmath>
#include <algorithm>
#include <cstdlib>
#include <stdio.h>
#include <vector>
#include <numeric>
#include <set>
#include <map>
#include <queue>
#define inf 9999999
using namespace std;

map <int,int> vis;
vector <vector <pair <int,int> > > adj(100);
vector <int> dist(100);

void dijkstra(int u,int t)
{
    priority_queue <pair <int,int> > q;
    int b,w,i;
    q.push({0,u});
    vis[u]=1;
    
    while(!q.empty())
    {
        u = q.top().second; q.pop();
        
        if(!vis[u])
        {
            vis[u]=1;
            for(i=0;i<adj[u].size();i++)
            {
                b = adj[u][i].first; w = adj[u][i].second;
                if(dist[b]>dist[u]+w)
                {
                    dist[b] = dist[u] + w;
                    q.push({-dist[b],b});
                }
            }
        }
    }
    
    cout << dist[t];
}


int main()
{
    int i,j,k,n,m,x,y,w,t;
    
    cin >> n >> m;
    for(i=0;i<m;i++)
    {
        cin >> x >> y >> w;
        adj[x].push_back({y,w});
        adj[y].push_back({x,w});
    }
    
    cin >> t;
    
    for(i=1;i<=n;i++)
    {
        dist[i]=inf;
    }
    
    dijkstra(1,t);
}

Answer 1

Mistake:

Your code is not working due to vis[u] = 1 which never lets you enter the for loop.

Actually there is no need of having a visited array in Dijkstra algorithm (atleast for this case). But using visited array here will decrease the time complexity and when there are negative edges it will be tricky, so be careful there.

#include <iostream>
#include <vector>
#include <queue>
#define inf 9999999

std::vector <std::vector <std::pair <int,int> > > adj(100);
std::vector <int> dist(100);

void dijkstra(int u,int t)
{
    std::priority_queue <std::pair<int, int>> q;
    int b, w, i;

    q.push({0, u});
    while(!q.empty()) {
        u = q.top().second; 
        q.pop();
        for(i = 0; i < adj[u].size(); i++) {
            b = adj[u][i].first; w = adj[u][i].second;
            if(dist[b] > dist[u] + w) {
                dist[b] = dist[u] + w;
                q.push({-dist[b], b});
            }
        }
    }
    
    std::cout << dist[t];
}


int main()
{
    int i, n, m, x, y, w, t;
    
    std::cin >> n >> m;
    for(i = 0;i < m; i++) {
        std::cin >> x >> y >> w;
        adj[x].push_back({y, w});
        adj[y].push_back({x, w});
    }
    
    std::cin >> t;
    
    for(i = 0; i < n; i++) {
        dist[i] = inf;
    }
     
    // making the source distance to 0
    dist[0] = 0;
    
    dijkstra(0,t);
}

Answer 2

The problem is vis[u]=1; just after q.push({0,u});.

Due to that, the check if(!vis[u]) cannot be passed and no nodes will be processed.

You should do dist[u]=0; instead of that.