C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MAXSIZE 2010
int n, m, dist[MAXSIZE], cnt[MAXSIZE];
int head, tail, queue[MAXSIZE], inQueue[MAXSIZE];
struct Edge {
    int to_, weight_;
    struct Edge *nxt_;
} *edges[MAXSIZE];

void init() {
    int x, y, z;
    scanf("%d%d", &n, &m);
    while (m--) {
        scanf("%d%d%d", &x, &y, &z);
        struct Edge *s = (struct Edge*) calloc(1, sizeof(struct Edge));
        s->to_ = y;
        s->weight_ = z;
        s->nxt_ = edges[x];
        edges[x] = s;
    }
    memset(dist, 0x3f, sizeof(dist));
    head = 0, tail = 0;
}

int empty() {
    return head == tail ? 1 : 0;
}

void push(int val) {
    queue[tail] = val;
    tail = (tail+1) % MAXSIZE;
}

void pop() {
    head = (head+1) % MAXSIZE;
}

int front() {
    return queue[head];
}

int SPFA() {
    for (int i = 1; i <= n; ++i) {
        push(i);
        inQueue[i] = 1;
        ++cnt[i];
    }

    while (!empty()) {
        int u = front();
        pop();
        inQueue[u] = 0;

        struct Edge *p = edges[u];
        while (p != NULL) {
            int v = p->to_, w = p->weight_;
            p = p->nxt_;
            if (dist[u] + w < dist[v]) {
                dist[v] = dist[u] + w;
                if (!inQueue[v]) {
                    push(v);
                    inQueue[v] = 1;
                    ++cnt[v];
                    if (cnt[v] >= n) {
                        return 0;
                    }
                }
            }
        }
    }

    return 1;
}

int main() {
    init();
    if (SPFA()) {
        puts("No");
    } else {
        puts("Yes");
    }

    return 0;
}

C++

#include <cstring>
#include <iostream>
#include <queue>
using namespace std;

const int MAXSIZE = 2010;
int n, m, dist[MAXSIZE], cnt[MAXSIZE];
bool inQueue[MAXSIZE];
queue<int> q;
struct Edge {
    int to_, weight_;
    Edge *nxt_;
    Edge(int to, int weight) : to_(to), weight_(weight), nxt_(nullptr) {}
} *edges[MAXSIZE];

void addEdge(int from, int to, int weight) {
    Edge *s = new Edge(to, weight);
    s->nxt_ = edges[from];
    edges[from] = s;
}

void init() {
    int x, y, z;
    cin >> n >> m;
    while (m--) {
        cin >> x >> y >> z;
        addEdge(x, y, z);
    }
    memset(dist, 0x3f, sizeof(dist));
}

bool SPFA() {
    for (int i = 1; i <= n; ++i) {
        q.push(i);
        inQueue[i] = true;
        ++cnt[i];
    }

    while (!q.empty()) {
        int u = q.front();
        q.pop();
        inQueue[u] = false;

        Edge *p = edges[u];
        while (p != nullptr) {
            int v = p->to_, w = p->weight_;
            p = p->nxt_;
            if (dist[u] + w < dist[v]) {
                dist[v] = dist[u] + w;
                if (!inQueue[v]) {
                    q.push(v);
                    inQueue[v] = true;
                    ++cnt[v];
                    if (cnt[v] >= n) {
                        return false;
                    }
                }
            }
        }
    }

    return true;
}

int main() {
    init();
    if (SPFA()) {
        cout << "No" << endl;
    } else {
        cout << "Yes" << endl;
    }

    return 0;
}