Whats rong in my code not passing all test cases

#1 
#include<iostream>
#include<string>
using namespace std;
bool checkcb(long long sub)
{
  if (sub==0||sub==1)
  {
      return false;
  }
  int a[]={2,3,5,7,11,13,17,19,23,29};
  for (int i = 0; i < sizeof(a)/sizeof(int); i++)
  {
      if (a[i]==sub)
      {
          return true;
      }
     
      
  }
  for (int i = 0; i <  sizeof(a)/sizeof(int); i++)
  {
      if (sub%a[i]==0)
      {
          return false;
      }
      
  }
  
  return true;



}
bool isvalid(int start,int end,bool visited[])
{
for (start; start<end; start++)
{
    if (visited[start]==true)
    {
        return false;
        /* code */
    }
    
}


return true;


}
int main(){
     int n;
     cin>>n;
    string s;
    cin>>s;
    int ans=0;
    bool visited[17]={false};
    for (int i = 0; i <=s.length()-1; i++)
    {
        for (int j = 1; j <=s.length()-i; j++)
        {
            string sub=s.substr(i,j);
         
             if (checkcb(stoll(sub))&&isvalid(i,i+j,visited))
            {
                ans++;
                for (int k = i; k<i+j; k++)
                {
                    visited[k]=true;
                }
            
            } 
            
        }
    }
    cout<<ans;
    return 0;
}
#2

hello @sagar_aggarwal

there is small logical .
what u r doing is from index i u r iterating all possible substring startng from i. this will not give maximum.

the optimal way is to first find all one length cb number then all 2 length cb number ,then all 3 length cb no and so on (why small lenght to big lenght ? it is becuase we want to maximize count of cb number which can be possible by picking small lenght cb number first).

image

#3

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