参考答案：

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

using namespace std;

const int N = 100010;
int e[N], ne[N], h[N], w[N], idx;
int dis[N];
int st[N], cnt[N];
int n, m;

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

bool spfa()
{
    memset(dis, 0x3f, sizeof(dis));
    dis[1] = 0;

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

    while (q.size())
    {
        int t = q.front();
        q.pop();
        st[t] = 0;
        for (int i = h[t]; i != -1; i = ne[i])
        {
            int j = e[i];
            if (dis[j] > dis[t] + w[i])
            {
                dis[j] = dis[t] + w[i];
                cnt[j] = cnt[t] + 1;
                if (cnt[j] >= n) return true;
                if (!st[j])
                {
                    q.push(j);
                    st[j] = 1;
                }
            }
        }
    }

    return false;
}
int main()
{
    cin >> n >> m;
    memset(h, -1, sizeof(h));

    for (int i = 1; i <= m; ++i)
    {
        int a, b, c;
        cin >> a >> b >> c;
        add(a, b, c);
    }

    if (spfa()) cout << "Yes";
    else cout << "No";

    return 0;
}