高斯消元

void solve(){
    rep(i,1,n+1){
        int t=i;
        rep(j,i+1,n+1) if(fabs(a[j][i])>fabs(a[t][i])) t=j;
        if(t!=i) rep(j,1,n+2) swap(a[i][j],a[t][j]);
        rep(k,i+1,n+1){
            double p=a[k][i]/a[i][i];
            rep(j,i,n+2) a[k][j]-=p*a[i][j];
        }
    }
    for(int i=n+1;i;i--){
        rep(j,i+1,n+1) a[i][n+1]-=a[j][n+1]*a[i][j];
        a[i][n+1]/=a[i][i];
    }
    rep(i,1,n+1){
        printf("%.3lf",a[i][n+1]);
        if(i!=n) putchar(' ');
    }
}