SPFA算法（用队列优化的Bellman_Ford算法）

#include <iostream>
#include <cstring>
#include <algorithm>
#include <queue>

using namespace std;

const int N = 100010;    

int n, m;
int e[N], ne[N], h[N], w[N], idx;
int dist[N];
bool st[N];

void add(int a,int b, int c)
{
    e[idx] = b;
    w[idx] = c;    
    ne[idx] = h[a];
    h[a] = idx ++;
}


int spfa()
{
    memset(dist, 0x3f, sizeof dist);
    dist[1] = 0;

    queue<int> q;
    q.push(1);
    st[1] = 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])
                {
                    q.push(j);
                    st[j] = true;
                }
            }
        }
    }


    return dist[n];
}

int main()
{
    memset(h, -1, sizeof h);

    scanf("%d%d", &n, &m);

    for (int i = 0; i < m; i ++)
    {
        int a, b, c;
        cin >> a >> b >> c;
        add(a, b, c);
    }

    int res = spfa();

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

    return 0;
}