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