HDU 4418 Time travel

#pragma comment(linker, "/STACK:1024000000,1024000000")
#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
#include<vector>
#include<map>
#include<set>
#include<queue>
#include<stack>
#include<iostream>
using namespace std;
typedef long long LL;
const double pi=acos(-1.0),eps=1e-10;
void File()
{
    freopen("D:\\in.txt","r",stdin);
    freopen("D:\\out.txt","w",stdout);
}
template <class T>
inline void read(T &x)
{
    char c = getchar();
    x = 0;
    while(!isdigit(c)) c = getchar();
    while(isdigit(c))
    {
        x = x * 10 + c - ‘0‘;
        c = getchar();
    }
}

int T,n,m,y,x,d,len;
int L[500];
double p[500];
bool f[500];

int const maxn = 500;
double a[maxn][maxn],ans[maxn];
bool free_x[maxn];

int sgn(double x)
{
    return (x>eps)-(x<-eps);
}
int gauss()
{
    int i, j, k;
    int max_r;
    int col;
    double temp;
    int free_x_num;
    int free_index;
    int equ = len,var = len;
    col = 0;
    memset(free_x,true,sizeof(free_x));
    memset(ans,0,sizeof ans);
    for (k = 0; k < equ && col < var; k++, col++)
    {
        max_r = k;
        for (i = k + 1; i < equ; i++)
        {
            if (sgn(fabs(a[i][col]) - fabs(a[max_r][col]))>0) max_r = i;
        }
        if (max_r != k)
        {
            for (j = k; j < var + 1; j++) swap(a[k][j], a[max_r][j]);
        }
        if (sgn(a[k][col]) == 0 )
        {
            k--; continue;
        }
        for (i = k + 1; i < equ; i++)
        {
            if (sgn(a[i][col])!=0)
            {
                double t = a[i][col] / a[k][col];
                for (j = col; j < var + 1; j++)
                {
                    a[i][j] = a[i][j] - a[k][j] * t;
                }
            }
        }
    }
    for(i=k;i<equ;i++)
    if(sgn(a[i][col])!=0) {return 0;}
    if (k < var)
    {
        for (i = k - 1; i >= 0; i--)
        {
            free_x_num = 0;
            for (j = 0; j < var; j++)
            {
                if ( sgn(a[i][j])!=0 && free_x[j]){
                    free_x_num++, free_index = j;
                }
            }
            if(free_x_num>1)    continue;
            temp = a[i][var];
            for (j = 0; j < var; j++)
            {
                if (sgn(a[i][j])!=0 && j != free_index) temp -= a[i][j] * ans[j];
            }
            ans[free_index] = temp / a[i][free_index];
            free_x[free_index] = 0;
        }
        return var - k;
    }

    for (i = var - 1; i >= 0; i--)
    {
        temp = a[i][var];
        for (j = i + 1; j < var; j++)
        {
            if (sgn(a[i][j])!=0) temp -= a[i][j] * ans[j];
        }
        ans[i] = temp / a[i][i];
    }
    return 1;
}

void bfs()
{
    memset(f,0,sizeof f);
    queue<int>q;
    q.push(x); f[x]=1;
    while(!q.empty())
    {
        int x = q.front(); q.pop();
        for(int i=1;i<=m;i++)
        {
            if(p[i]<eps) continue;
            if(f[(x+i)%len]==0)
            {
                f[(x+i)%len]=1;
                q.push((x+i)%len);
            }
        }
    }
}

int main()
{
    scanf("%d",&T);
    while(T--)
    {
        scanf("%d%d%d%d%d",&n,&m,&y,&x,&d);
        for(int i=1;i<=m;i++)
        {
            int x; scanf("%d",&x);
            p[i]=1.0*x/100;
        }

        for(int i=0;i<n;i++) L[i]=i; len=n;
        int tmp=n-2 ,pos = n; while(tmp>=1) L[pos++]=tmp--;
        len=pos;

       // for(int i=0;i<len;i++) printf("%d ",L[i]); printf("\n");

        if(d==1)
        {
            for(int i=n;i<=2*(n-1);i++) if(L[i]==x) { x=i; break; }
        }

        bfs();

        bool fail=1;
        for(int i=0;i<len;i++) if(L[i]==y&&f[i]==1) fail=0;

        if(fail==1)
        {
            printf("Impossible !\n");
            continue;
        }

        memset(a,0,sizeof a);

        for(int i=0;i<len;i++)
        {
            if(L[i]==y) continue;
            a[i][i]=-1;
            for(int j=1;j<=m;j++)
            {
                if(L[(i+j)%len]==y||f[(i+j)%len]==0) a[i][(i+j)%len]=0;
                else a[i][(i+j)%len]+=p[j];
                a[i][len]-=p[j]*j;
            }
        }

        gauss();

      //  for(int i=0;i<len;i++) printf("%lf  ",ans[i]);
      //  printf("\n");

        printf("%.2f\n",ans[x]);
    }
    return 0;
}