AcWing 852. spfa判断负环 边权可能为负数。

#include <cstring>
#include <iostream>
#include <algorithm>
#include <queue>
using namespace std;
const int N = 2010, M = 10010;
int n, m;
int h[N], w[M], e[M], ne[M], idx;
int dist[N];//最短距离 
int cnt[N];//当前最短路的边数，如果大于等于n，那么有负环 
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 ++ ) {
        st[i] = true;//可能负环1到不了，那么就把所有的点加进去 
        q.push(i);
    }
    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() {
    scanf("%d%d", &n, &m);
    memset(h, -1, sizeof h);
    while (m -- ) {
        int a, b, c;
        scanf("%d%d%d", &a, &b, &c);
        add(a, b, c);
    }
    if (spfa()) puts("Yes");
    else puts("No");
    return 0;
}