题解 | #公交线路#

采用spfa算法实现

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

using namespace std;

const int N = 1010, M = 2000010, INF = 1000000000;

int n, m,s,t;
int dist[N], q[N];      // dist表示每个点到起点的距离, q 是队列
int h[N], e[M], v[M], ne[M], idx;       // 邻接表
bool st[N];     // 存储每个点是否在队列中

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

void spfa()
{
    int hh = 0, tt = 0;
    memset(dist, 0x3f, sizeof dist);
    
    queue<int> q;
    dist[s] = 0;
    q.push(s);
    st[s] = 1;
    while (!q.empty())
    {
        int t = q.front();
        q.pop();
        st[t] = 0;
        for (int i = h[t]; i != -1; i = ne[i]){
            int j=e[i];
            if (dist[j] > dist[t] + v[i])
            {
                dist[j] = dist[t] + v[i];
                if (!st[j])
                {
                    st[j] = 1;
                    q.push(j);
                }
            }
        }
    }
}

int main()
{
    memset(h, -1, sizeof h);
    cin >> n >> m>>s>>t;
    for (int i = 0; i < m; i++)
    {
        int a, b, c;
        cin >> a >> b >> c;
        add(a, b, c);
        add(b, a, c);
    }
    spfa();
    if(dist[t]>=0x3f3f3f3f) cout<<-1<<endl;
    else
    cout << dist[t] << endl;
    return 0;
}