const long long int MOD = 1e9 + 7;
        
        This is used to be under long long int.

int -> 31 bit for numbers. unsigned int -> 32 bit for numbers. long long int -> 64 bit.

<aside> 💡 Sometimes, you will only use positive numbers. And you will be instructed, “answer will not exceed 32 bit integer number”. In that case, you have to use unsigned int.

Using only int will get you overflown. And you don’t usually use MOD to be under 32 bit, MOD is used to be under 64 bit as shown above. Using long long might not work in cases where you have to return int from the function you are working with, but the intermediate calculation requires all those 32 bits.

</aside>