I don't know where this code is wrong. I modularized it, but the answer is different.

static long mod(long x){
    while(x<0)
        x+=1000000007;
    return (x%1000000007);    
}
public static long countArray(int n, int k, int x)
{
// Return the number of ways to fill in the array.
    if(k<=1 || n<2)return 0;
    if(k==2 && n<=3) return 0;
    if(k==2)
    {
        if(x==1 ) 
        {
            if( n%2==0)  return 0;
            return 1;
        }
        if(x==2)
        {
            if( n%2==1)  return 0;
            return 1;
        }
    }

    long result= 1;
    int N=n-2;

    for(int i=0;i<N;i++)
    {
        result = mod(result*(k-1));
    }
    //result=mod(result*k);
    Console.WriteLine("result : "+result);
    long kn=1;
    for(int i=0;i<N-1;i++)
    {
        kn = mod(kn*(k-1));
    }
    Console.WriteLine("kn : "+kn);
    //result=mod(result-2*kn);
    Console.WriteLine("result : "+result);
    long ss1=0;
    long ss2=0;

        ss1 =mod(((kn+(k-1)*(((N-1)%2==0)?1:-1))/k));
        ss2 =mod(((kn+(((N)%2==0)?1:-1))/k));
    if(x==1)
    {
        kn=mod(ss2*(k-1));
    Console.WriteLine("ss1 : "+kn);
    }
    else 
    {
        kn=mod(ss2*(k-2)+ss1);
    Console.WriteLine("ss2 : "+kn);
    }
    return mod(result -kn);
}