spfa求最短路 -2025/11/26

spfa求最短路

#include <bits/stdc++.h>
using namespace std;

const int N = 150010;
typedef pair<int,int> PII;
int e[N], h[N], ne[N], w[N], idx;
int n, m;
int dist[N];
int st[N];

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

// spfa里的st存储的是 队列里是否有这个元素
// dijkstra 算法里的st存的是这个点是否已经访问过, 已经是最小路径了
int spfa(){
	memset(dist, 0x3f, sizeof dist);
	dist[1] = 0;
	queue<int> q;
	q.push(1);
	while(q.size()){
		auto 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;
				}
			}
		}
	}
	
	if(dist[n] == 0x3f3f3f3f) return 0x3f3f3f3f;
	return dist[n];
}

int main(){
	ios::sync_with_stdio(false), cin.tie(0), cout.tie(0);
	memset(h, -1, sizeof h);
	cin >> n >> m;
	
	for(int i  = 1; i <= m; i++){
		int a, b, c;
		cin >> a >> b >> c;
		add(a, b, c);
	}
	
	int ans = spfa();
	
	if(ans == 0x3f3f3f3f) puts("impossible");
	else cout << ans << "\n";
	return 0;
}