2022.6.30-----leetcode.1175

    //统计质数，计算排列数
    int mod=(int)1e9+7;
    public int numPrimeArrangements(int n) {
        int cnt=0;
        for(int i=2;i<=n;i++){
            if(isPrime(i))
                cnt++;
        }
        return (int)((factorial(cnt)%mod)*(factorial(n-cnt)%mod)%mod);
    }
    boolean isPrime(int num){
        if (num == 0||num==1)
            return false;
        if (num == 2 || num == 3)
            return true;
        if (num % 6 != 1 && num % 6 != 5)
            return false;
        int end = (int) Math.pow(num,1.0/2);
        for (int i = 5; i <= end; i += 6) {
            if (num % i == 0 || num % (i + 2) == 0)
                return false;
        }
        return true;
    }
    long factorial(int n){
        if(n==0)
            return 1;
        return factorial(n-1)*n%mod;
    }