Find the highest occurring digit in prime numbers in a range

#include <bits/stdc++.h>
using namespace std;
//This question is done aready.........................
int main() {
	//code
	int n,a,b;cin>>n;
	int arr[1000001]={0};
	for(int i=0;i<1000000;i++){
	        int k=i;
	        k+=i;
	        while(k<=1000000){
	            arr[k]=1;
	            k+=i;
	        }
	}
	while(n--){
	    cin>>a>>b;
	    string h="";
	    for(int i=a;i<=b;i++){
	        if(arr[i]==0){h+=to_string(i);}
	    }
	    map<int,int> m;
	    for(int i=0;i<h.length();i++){
	        m[(h[i]-'0')]++;
	    }
	    int ma=0,d=0;
	    for(auto e: m){
	        if(e.second>=ma){ma = e.second; d = e.first;}
	    }
	    cout<<d<<endl;
	}
	return 0;
}