Ultraviolet A 11090 Going in Cycle !! (Binary + spfa)

Returns a directed positive right edge graph with n vertices and m edges, and returns the loop with the smallest average value.

The binary mid value. Subtract the mid value from each edge to check whether a negative ring exists. But pay attention to this question... The source image is not necessarily strongly connected, so the general spfa cannot be used to determine the negative ring. To start the phase in spfa, each point must be added to the queue once. Make sure that each strongly connected component can be added.

 

#include<algorithm>#include<iostream>#include<cstring>#include<cstdlib>#include<fstream>#include<sstream>#include<bitset>#include<vector>#include<string>#include<cstdio>#include<cmath>#include<stack>#include<queue>#include<stack>#include<map>#include<set>#define FF(i, a, b) for(int i=a; i<b; i++)#define FD(i, a, b) for(int i=a; i>=b; i--)#define REP(i, n) for(int i=0; i<n; i++)#define CLR(a, b) memset(a, b, sizeof(a))#define debug puts("**debug**")#define LL long long#define PB push_backusing namespace std;const int maxn = 51;const double INF = 1000000000;struct Edge{    int from, to;    double dist;};int n, m;vector<Edge> edges;vector<int> G[maxn];bool inq[maxn];double d[maxn];int cnt[maxn];inline void init(){    REP(i, n) G[i].clear(); edges.clear();}void add(int a, int b, double c){    edges.PB((Edge){a, b, c});    int nc = edges.size();    G[a].PB(nc-1);}bool negacycle(){    queue<int> q;    CLR(inq, 0); CLR(cnt, 0);    REP(i, n)   d[i] = INF, q.push(i);    while(!q.empty())    {        int u = q.front(); q.pop();        inq[u] = false;        int nc = G[u].size();        REP(i, nc)        {            Edge e = edges[G[u][i]];            if(d[e.to] > d[u] + e.dist)            {                d[e.to] = d[u] + e.dist;                if(!inq[e.to])                {                    q.push(e.to);                    inq[e.to] = 1;                    if(++cnt[e.to] > n) return true;                }            }        }    }    return false;}bool ok(double m){    int nc = edges.size();    REP(i, nc) edges[i].dist -= m;    bool ret = negacycle();    REP(i, nc) edges[i].dist += m;    return ret;}int main(){    int T;    scanf("%d", &T);    FF(kase, 1, T+1)    {        scanf("%d%d", &n, &m);        init();        int a, b; double c;        double L = 0, R = 0, M;        while(m--)        {            scanf("%d%d%lf", &a, &b, &c); a--; b--;            add(a, b, c);            R = max(R, c);        }        printf("Case #%d: ", kase);        if(!ok(R + 1))        {            puts("No cycle found.");            continue;        }        while(R - L > 1e-3)        {            M = (L + R) / 2;            if(ok(M)) R = M;            else L = M;        }        printf("%.2lf\n", R);    }    return 0;}