POJ 3532 Resistance 高斯消元

#include <cstdio>
#include <algorithm>
#include <cstring>
#include <cmath>

using namespace std;

int N,M,u,v;
const int MAX = 1000;
const double EPS = 1e-6;
double a[MAX][MAX];
//a is a matrix , l is represent free variable, ans store the answer

double gauss() {
int n = N;
for (int i = 0; i < n; i++) {
int r;
for (r = i; r < n; r++)
if (fabs(a[r][i]) >= EPS)
break;
if (r == n) continue;
for (int j = 0; j <= n; j++) swap(a[i][j], a[r][j]);
for (int j = n; j > i; j--) a[i][j] /= a[i][i];
a[i][i] = 1.0;
for (int j = 0; j < n; j++) {
if (i == j) continue;
if (fabs(a[j][i]) >= EPS) {
for (int k = n; k > i; k--)
a[j][k] -= a[j][i] * a[i][k];
a[j][i] = 0.0;
}
}
}
return a[0]
/ a[0][0];
}

int main(void)
{
//freopen("input.txt","r",stdin);
double r;
scanf("%d%d",&N,&M);
for(int i = 0; i < M; ++i){
scanf("%d%d%lf",&u,&v,&r);
--u,--v;
double s = 1.0 / r;
a[u][u] += s;
a[v][v] += s;
a[u][v] -= s;
a[v][u] -= s;
}
N++;
a[0]
= 1.0;
a[N-2]
= -1.0;
a[N-1][N-2] = 1;
printf("%.2f\n",gauss());
return 0;
}