spfa判断负环

Posted 2021-05-21 20203246

#include <bits/stdc++.h>
using namespace std;
const int maxn=1e5+10;
struct node{
    int next,to,val;
}e[maxn<<1];
int head[maxn<<1],k;
void add(int from,int to,int val){
    e[++k]={ head[from],to,val };
    head[from]=k;
}
int vis[maxn],dis[maxn<<1],cnt[maxn<<1];
int n,m;
bool spfa(){
    queue<int>q;
    for(int i=1;i<=n;i++){
        q.push(i);vis[i]=true;
    }
    while(!q.empty()){
        int t=q.front();q.pop();
        vis[t]=false;
        for(int i=head[t];i;i=e[i].next){
            int x=e[i].to,y=e[i].val;
            if(dis[x]>dis[t]+y){
                dis[x]=dis[t]+y;
                cnt[x]=cnt[t]+1;
                if(cnt[x]>n-1)return true;
                if(!vis[x]){
                    q.push(x);
                    vis[x]=true;
                }
            }
        }
    }
    return false;
}
int main()
{
    ios::sync_with_stdio(false);
    cin>>n>>m;
    while(m--){
        int u,v,val;
        cin>>u>>v>>val;
        add(u,v,val);
    }
    if(spfa())cout<<"Yes";
    else cout<<"No";
    return 0;
}