|
|
|
|
|
#include <bits/stdc++.h>
|
|
|
|
|
|
|
|
|
|
|
|
using namespace std;
|
|
|
|
|
|
const int N = 1010, M = 5010;
|
|
|
|
|
|
double dist[N];
|
|
|
|
|
|
bool st[N];
|
|
|
|
|
|
|
|
|
|
|
|
int idx, h[N], e[M], w[M], ne[M];
|
|
|
|
|
|
void add(int a, int b, int c) {
|
|
|
|
|
|
e[idx] = b, w[idx] = c, ne[idx] = h[a], h[a] = idx++;
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
// ① dfs找负环的标准模板
|
|
|
|
|
|
bool dfs1(int u) {
|
|
|
|
|
|
if (st[u]) return 1;
|
|
|
|
|
|
bool flag = 0;
|
|
|
|
|
|
st[u] = 1;
|
|
|
|
|
|
for (int i = h[u]; ~i; i = ne[i]) {
|
|
|
|
|
|
int v = e[i];
|
|
|
|
|
|
if (dist[v] > dist[u] + w[i]) {
|
|
|
|
|
|
dist[v] = dist[u] + w[i];
|
|
|
|
|
|
flag = dfs1(v);
|
|
|
|
|
|
if (flag) break;
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
// 回溯写法
|
|
|
|
|
|
st[u] = 0;
|
|
|
|
|
|
return flag;
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
// ② 标准错误答案
|
|
|
|
|
|
bool dfs2(int u) {
|
|
|
|
|
|
if (st[u]) return 1;
|
|
|
|
|
|
bool flag = 0;
|
|
|
|
|
|
st[u] = 1;
|
|
|
|
|
|
for (int i = h[u]; ~i; i = ne[i]) {
|
|
|
|
|
|
int v = e[i];
|
|
|
|
|
|
if (dist[v] > dist[u] + w[i]) {
|
|
|
|
|
|
cout << "u=" << u << ",v=" << v << endl;
|
|
|
|
|
|
dist[v] = dist[u] + w[i];
|
|
|
|
|
|
flag = dfs2(v);
|
|
|
|
|
|
if (flag) return 1;
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
// 坚决不回溯
|
|
|
|
|
|
|
|
|
|
|
|
return flag;
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
/*
|
|
|
|
|
|
测试用例：
|
|
|
|
|
|
4 4
|
|
|
|
|
|
1 2 -2
|
|
|
|
|
|
2 3 -6
|
|
|
|
|
|
2 4 1
|
|
|
|
|
|
4 3 -4
|
|
|
|
|
|
|
|
|
|
|
|
结论：图中是没有负环的，应该返回0
|
|
|
|
|
|
*/
|
|
|
|
|
|
int main() {
|
|
|
|
|
|
#ifndef ONLINE_JUDGE
|
|
|
|
|
|
freopen("361.in", "r", stdin);
|
|
|
|
|
|
#endif
|
|
|
|
|
|
int n, m;
|
|
|
|
|
|
scanf("%d %d", &n, &m);
|
|
|
|
|
|
// 初始化邻接表
|
|
|
|
|
|
memset(h, -1, sizeof h);
|
|
|
|
|
|
int a, b, c;
|
|
|
|
|
|
for (int i = 0; i < m; i++) {
|
|
|
|
|
|
scanf("%d %d %d", &a, &b, &c);
|
|
|
|
|
|
add(a, b, c);
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
cout << dfs1(1) << endl; // 返回正确解0，表示没有找到负环
|
|
|
|
|
|
|
|
|
|
|
|
// 如果按不回溯，而是memset的办法，结果就是错误的啦~
|
|
|
|
|
|
memset(st, 0, sizeof st);
|
|
|
|
|
|
memset(dist, 0, sizeof dist);
|
|
|
|
|
|
cout << dfs2(1) << endl; // 返回错误结果1，表示找到负环
|
|
|
|
|
|
return 0;
|
|
|
|
|
|
}
|
