如代码，只拿了40分……蒟蒻求质数判断求助

#include <iostream>
#include <cstring>
using namespace std;
int main (){
	string a;
	int num[123] = {0}, maxn = 0, minn = 100, bit;
	bool isPrime = 1;
	cin >> a;
	for(int i = 0; i <= a.size(); i++){
		for(int j = 97; j <= 122; j++)
		if(a[i] == j){
			num[j]++;
		}
	}
	for(int k = 97; k <= 122; k++){
		if(num[k] > maxn){
			maxn = num[k];	
		}
		if(num[k] < minn){
			minn = num[k] + 1;	
		}
	}
	if(maxn == a.size()){
		minn = 0;	
	}
	bit = maxn - minn; 
	for(int j = 2; j <= bit / 2; j++){
      	if(bit % j == 0){
          	isPrime = 0;
        break;
      	}
      	if (bit == 0){
      		isPrime = 0;
      		
		}
	}
	if (isPrime == 1){
		cout << "Lucky Word";
	}
  	if (isPrime == 0){
      	cout << "No Answer";
	}
    cout << endl << maxn - minn;
  	return 0;	 
}