如代码，蒟蒻求助关于质数判断的问题……

#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;    
}