不知道问题在哪里

#include<iostream>
#include<algorithm>
#include<queue>
#include<cstring>
using namespace std;
const int N=100010;
int h[N],e[N],ne[N],idx,w[N];
int dist[N];
bool st[N];
int n,m,s;

void add(int x,int y,int z)
{
    e[idx]=y;
    ne[idx]=h[x];
    w[idx]=z;
    h[x]=idx++;
    return ;
}

int spfa()
{
   memset(dist, 0x3f, sizeof dist);
    dist[s] = 0;

    queue<int> q;
    q.push(s);
    st[s] = true;
    
    while(q.size())
    {
        int t=q.front();
        q.pop();
        st[t]=false;
        
        for(int i=h[t];i!=-1;i=ne[i])
        {
            int j=e[i];
            if(dist[j]>dist[t]+w[i])
            {
                dist[j]=dist[t]+w[i];
                if(!st[j])
                {
                    st[j]=true;
                    q.push(j);
                }
            }
        }
    }
    
    return dist[n];
}
int main()
{
    scanf("%d%d", &n, &m);
	cin>>s;
    memset(h, -1, sizeof h);

    while (m -- )
    {
        int a, b, c;
        scanf("%d%d%d", &a, &b, &c);
        add(a, b, c);
    }

    int t = spfa();

    if (t == 0x3f3f3f3f) puts("impossible");
    else printf("%d\n", t);

    return 0;
}