spfa判断负环

#include <cctype>
#include <cstdio>
#include <cstring>
using namespace std;

const int maxn=2e5+5, maxm=2e5+5;

int getint(){
    char c; int flag=1, re=0;
    for (c=getchar(); !isdigit(c); c=getchar())
        if (c=='-') flag=-1;
    for (re=c-48; c=getchar(), isdigit(c); re=re*10+c-48);
    return re*flag;
}

struct Graph{
    struct Edge{
        int to, next, v; Graph *bel;
        inline int operator *(){ return to; }
        Edge& operator ++(){
            return *this=bel->edge[next]; }
    };
    void reset(){
        cntedge=0; memset(fir, 0, sizeof(fir)); }
    void addedge(int x, int y, int v){
        Edge &e=edge[++cntedge];
        e.to=y; e.next=fir[x]; e.v=v;
        e.bel=this; fir[x]=cntedge;
    }
    Edge& getlink(int x){ return edge[fir[x]]; }
    Edge edge[maxm*2];
    int cntedge, fir[maxn];
}g;

int T, n, m, dis[maxn], visit[maxn]; bool flag;

void spfa(int now){
    Graph::Edge e=g.getlink(now);
    visit[now]=1; if (flag) return;
    for (; *e; ++e)
        if (dis[now]+e.v<dis[*e]){
            dis[*e]=dis[now]+e.v;
            if (visit[*e]) flag=true;
            else spfa(*e);
        }
    visit[now]=0;
}

int main(){
    T=getint(); int x, y, v;
    while (T--){
        g.reset(); flag=false;
        n=getint(); m=getint();
        for (int i=1; i<=m; ++i){
            x=getint(); y=getint(); v=getint();
            g.addedge(x, y, v);
            if (v>=0) g.addedge(y, x, v);
        }
        for (int i=1; i<=n; ++i) dis[i]=visit[i]=0; 
        //不能设置成inf，不然就退化成n^2算法了
        for (int i=1; i<=n; ++i) spfa(i); //不能判断是否访问过！
        if (flag) printf("YE5\n");
        else printf("N0\n");
    }
    return 0;
}