SPFA
#include <iostream>
#include <cstring>
#include <queue>
using namespace std;
const int N = 150010;
typedef pair<int, int> PII;
int h[N], e[N], ne[N], w[N], idx;
int n, m, dist[N], st[N];
int add(int a, int b, int c)
{
w[idx] = c;
e[idx] = b;
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] = 1; // 当前这个点是不是在队列当中
while (q.size())
{
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] + w[i])
{
dist[j] = dist[t] + w[i];
if (!st[j])
{
q.push(j);
st[j] = 1;
}
}
}
}
return dist[n];
}
int main()
{
cin >> n >> m;
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)
printf(“impossible”);
else
printf(“%d\n”, t);
return 0;
}
