SPFA续集（判断负环）

#include <iostream>
#include <cstring>
#include <algorithm>
#include <queue>

using namespace std;

const int N = 100010;    

int n, m;
int e[N], ne[N], h[N], w[N], idx;
int dist[N], cnt[N];
bool st[N];

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


int spfa()
{
    queue<int> q;
    for (int i = 1; i <= n; i ++ )
    {
        q.push(i);
        st[i] = true;
    }

    while(q.size())
    {
        int t = q.front();
        q.pop();

        st[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];
                cnt[j] = cnt[t] + 1;
                if(cnt[j] >= n) return true;
                if(!st[j])
                {
                    q.push(j);
                    st[j] = true;
                }
            }
        }
    }


    return false;
}

int main()
{
    memset(h, -1, sizeof h);

    scanf("%d%d", &n, &m);

    for (int i = 0; i < m; i ++)
    {
        int a, b, c;
        cin >> a >> b >> c;
        add(a, b, c);
    }

    int res = spfa();

    if (res) puts("Yes");
    else puts("No");

    return 0;
}

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