Calcular el dígito mayor de un numero C++

#include <iostream>
#include <math.h>
using namespace std;
int digitos(int, int);
int main(int argc, char *argv[]) {
int n, y;
bool flag=true;
while(flag==true)
{
cout<< "Introducir un numero mayor a 9"<<endl;
cin>>n;
if(n>9)
flag=false;
else
flag=true;
}
y=log(n)/log(10)+1;
cout<< "El digito mayor es : "<<digitos(n, y);

return 0;
}
int digitos(int x, int y)
{
int nh, d, b;
while(y>1)
{
nh=x%10;
x=x/10;
d=x%10;
if(nh>d)
{
x=x/10;
x=x*10+nh;
b=nh;
}
else if (d>nh)
{
x=x/10;
x=x*10+d;
b=d;
}
else if(nh==d)
b=nh;
y--;
}
return b;
}