spfa

//spfa algorithm template (adjacency matrix):
//c++ code:
void spfa(int s){
	for(int i=0; i<=n; i++) dis[i]=99999999; //Initialize the distance from each point i to s
	dis[s]=0; vis[s]=1; q[1]=s; Queue initialization, s is the starting point
	int i, v, head=0, tail=1;
	while (head<tail){ queue is not empty
		head++;
		v=q[head]; take the first element of the queue
		vis[v]=0; Release the first node of the team, because this node may be used to relax other nodes next time and re-enter the team
		for(i=0; i<=n; i++) for all vertices
		   if (a[v][i]>0 && dis[i]>dis[v]+a[v][i]){  
				dis[i] = dis[v]+a[v][i]; modify the shortest path
				if (vis[i]==0){ If the extension node i is not in the queue, enter the queue
					tail++;
					q[tail]=i;
					show [i] = 1;
				}
		   }
		
	}
}

Reprinted from https://blog.csdn.net/xunalove/article/details/70045815

The above is the spfa algorithm of BFS, which may time out

DFS spfa

#include <iostream>
using namespace std;
int q[10001], dis[201], a[201][201], b[201][201];
bool fish [201];
int n, m, s, t;
bool flag=false;
void spfa(int s){
    if(flag) return;
    if(vis[s])//If there is an edge that has experienced twice, there is a negative cycle
    {
        flag=true;
        return;
    }
    else vis [s] = 1;
	for(int i=1; i<=b[s][0]; i++)
		if (dis[b[s][i]] > dis[s]+a[s][b[s][i]]){
				dis[b[s][i]] = dis[s]+a[s][b[s][i]];
				spfa(b[s][i]);
				if(flag) return;
		}

}
int main(){
	int x, y, z,w;
	int t;
	cin>>t;
	while(t--){
        flag=false;
	cin >> n >> m>> w;
	for(int i=0; i<m; i++){
		cin >> x >> y >> z;
		//if (a[x][y]!=0 && z>a[x][y]) continue;
		b[x][0]++; b[x][b[x][0]]=y; a[x][y]=z;
		b[y][0]++; b[y][b[y][0]]=x; a[y][x]=z;
	}
	for(int i=0;i<w;i++)
    {
        cin >> x >> y >> z;
		//if (a[x][y]!=0 && z>a[x][y]) continue;
		b[x][0]++; b[x][b[x][0]]=y; a[x][y]=-z;//b[x][0] means from vertex x How many edges are there from the start b[x][b[x][0]]=y stores the end point of the b[x][0]th edge starting from x
		b[y][0]++; b[y][b[y][0]]=x; a[y][x]=-z;
    }
	for(int i=0; i<=n; i++) dis[i]=99999999;
	dis[1]=0;
	spfa(s);
	if (flag) cout << "YES" << endl;
	else cout << "NO" << endl;
	}
	return 0;
}