HDU5006 Resistance(高斯消元)

#include <iostream>
#include <cstdio>
#include <cstring>
#include <string>
#include <algorithm>
#include <cmath>
#include <queue>
#include <cassert>
#include <vector>
using namespace std;

#define maxn 11000
#define eps 1e-7
struct Edge{
    int u,v,c;
    Edge(int ui,int vi,int ci):u(ui),v(vi),c(ci){}
    Edge(){}
};
int dcmp(double x){
    return (x>eps)-(x<-eps);
}
vector<Edge> E;
vector<int> G[maxn];
int n,m,s,t;
int bel[maxn];
int btot;

void dfs(int u,int mark)
{
    bel[u]=mark;
    for(int i=0;i<G[u].size();++i){
        int v=G[u][i];
        if(bel[v]) continue;
        else dfs(v,mark);
    }
}

void getBelong()
{
    btot=0;
    memset(bel,0,sizeof(bel));
    for(int i=1;i<=n;++i){
        if(!bel[i]){
            ++btot;
            dfs(i,btot);
        }
    }
}

int cnt[500][500];
double mat[500][500];

void gauss(int n)
{
    for(int i=1;i<=n;++i){
        int pivot=i;
        for(int j=i;j<=n;++j){
            if(abs(mat[j][i])>abs(mat[pivot][i])) pivot=j;
        }
        for(int x=1;x<=n+1;++x){
            swap(mat[i][x],mat[pivot][x]);
        }
        if(abs(mat[i][i])<eps) continue;
        for(int j=i+1;j<=n+1;++j){
            mat[i][j]/=mat[i][i];
        }
        for(int j=1;j<=n;++j){
            if(i!=j){
                for(int k=i+1;k<=n+1;++k){
                    mat[j][k]-=mat[j][i]*mat[i][k];
                }
            }
        }
    }
}




int main()
{
    int T;cin>>T;
    while(T--){
        scanf("%d%d%d%d",&n,&m,&s,&t);
        for(int i=0;i<=n;++i) G[i].clear();
        E.clear();
        int ui,vi,ci;
        for(int i=0;i<m;++i){
            scanf("%d%d%d",&ui,&vi,&ci);
            if(ci==0){
                G[ui].push_back(vi);
                G[vi].push_back(ui);
            }
            else{
                E.push_back(Edge(ui,vi,ci));
            }
        }
        getBelong();
        if(bel[s]==bel[t]){
            printf("%.6lf\n",0);continue;
        }
        memset(cnt,0,sizeof(cnt));
        memset(mat,0,sizeof(mat));
        for(int i=0;i<E.size();++i){
            ui=E[i].u;vi=E[i].v;
            if(bel[ui]!=bel[vi]){
                cnt[bel[ui]][bel[vi]]++;
                cnt[bel[vi]][bel[ui]]++;
            }
        }
        for(int i=1;i<=btot;++i){
            if(i==bel[s]){
                mat[i][i]=1.0;
                mat[i][btot+1]=1;
                continue;
            }
            else if(i==bel[t]){
                mat[i][i]=1.0;
                mat[i][btot+1]=0;
                continue;
            }
            for(int j=1;j<=btot;++j){
                if(i==j||cnt[i][j]==0) continue;
                mat[i][i]+=cnt[i][j];
                mat[i][j]-=cnt[i][j];
            }
        }
        gauss(btot);
        double I=0;
        for(int i=1;i<=btot;++i){
            if(i==bel[s]) continue;
            if(cnt[bel[s]][i]>0){
                I+=(1-mat[i][btot+1])*cnt[bel[s]][i];
            }
        }
        double R=1/I;
        if(dcmp(I-eps)==0) {
            printf("inf\n");
            continue;
        }
        printf("%.6lf\n",R);
    }
    return 0;
}