Given a directed graph with n points and m edges, there may be multiple edges and self-loops in the graph, and the edge weights may be negative.
Please find the shortest distance from point 1 to point n. If it is impossible to get from point 1 to point n, output impossible.
The data guarantees that there are no negative weight loops.
Input format
The first line contains integers n and m.
Each of the next m lines contains three integers x, y, z, indicating that there is a directed edge from point x to point y, and the edge length is z.
Output Format
Output an integer representing the shortest distance from point 1 to point n.
If the path does not exist, output impossible.
The data range is
1≤n, m≤105, and
the absolute value of the side length involved in the figure does not exceed 10000.
Input sample:
3 3
1 2 5
2 3 -3
1 3 4
Output sample:
2
#include<bits/stdc++.h>
using namespace std;
const int N = 100010;
int h[N], e[N], ne[N], w[N], idx;
int dist[N];
bool vis[N];
int n, m;
void add(int a, int b, int c)
{
e[idx] = b;
ne[idx] = h[a];
w[idx] = c;
h[a] = idx++;
}
int spfa()
{
memset(dist, 0x3f, sizeof dist);
dist[1] = 0;
queue<int> q;
q.push(1);
vis[1] = true;
while(q.size()) {
int t = q.front();
q.pop();
vis[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(!vis[j]) {
q.push(j);
vis[j] = true;
}
}
}
}
if(dist[n] == 0x3f3f3f3f) return -1;
else return dist[n];
}
int main()
{
memset(h, -1, sizeof h);
cin >> n >> m;
int a, c, b;
for(int i=0; i<m; i++) {
cin >> a >> b >> c;
add(a, b, c);
}
int t = spfa();
if(t == -1) puts("impossible");
else cout << t << endl;
return 0;
}