uva 11090 - Going in Cycle!!（二分+BellmanFord）

题目链接：uva 11090 - Going in Cycle!!

二分环的平均值，叫每条边减掉该值，判断是否有负环存在，存在则不行。

#include <cstdio>#include <cstring>#include <vector>#include <queue>#include <algorithm>using namespace std;const int maxn = 55;const double eps = 1e-3;struct BellmanFord {struct Edge {int u, v;double d;Edge(int u = 0, int v = 0, double d = 0):u(u), v(v), d(d) {}};int n, m;vector<Edge> edges;vector<int> G[maxn];bool inq[maxn];int p[maxn], c[maxn];double d[maxn];void init (int n) {this->n = n;edges.clear();for (int i = 0; i < n; i++) G[i].clear();}void addEdge (int u, int v, double d) {edges.push_back(Edge(u, v, d));m = edges.size();G[u].push_back(m-1);}bool negativeCycle() {queue<int> que;memset(c, 0, sizeof(c));memset(inq, 0, sizeof(inq));for (int i = 0; i < n; i++) {d[i] = 0, inq[i] = true;que.push(i);}while (!que.empty()) {int u = que.front();que.pop();inq[u] = false;for (int i = 0; i < G[u].size(); i++) {Edge& e = edges[G[u][i]];if (d[e.v] > d[u] + e.d) {d[e.v] = d[u] + e.d;p[e.v] = G[u][i];if (!inq[e.v]) {que.push(e.v);inq[e.v] = true;if (++c[e.v] > n) return true;}}}}return false;}}solver;int N, M, T;void init () {scanf("%d%d", &N, &M);solver.init(N);T = 0;int u, v, d;while (M--) {scanf("%d%d%d", &u, &v, &d);solver.addEdge(u-1, v-1, d);T = max(T, d);}}bool judge(double x) {for (int i = 0; i < solver.m; i++) solver.edges[i].d -= x;bool ret = solver.negativeCycle();for (int i = 0; i < solver.m; i++) solver.edges[i].d += x;return ret;}int main () {int cas;scanf("%d", &cas);for (int kcas = 1; kcas <= cas; kcas++) {init ();printf("Case #%d: ", kcas);if (!judge(T + 1)) printf("No cycle found.\n");else {double l = 0, r = T;while (r - l > eps) {double mid = (l + r) / 2;if (judge(mid)) r = mid;else l = mid;}printf("%.2lf\n", l);}}return 0;}