Through this library we can calculate Factorial, Exponentiation & GCD of invariably large numbers.

Step1) write #include"deque.h" in your cpp file.
Step2) To Use this see the example below
For Eg.,

BigInt bigint_one, bigint_two;
// Take two string for whom you want to calculate gcd, or exponentiation and one string for factorial.
cin>>string_one>>string_two;
//Intialize BigInt object with these strings
bingint_one.setInt(string_one);
bigint_second.setInt(string_two);
// Now the operation defined as "fact()" for factorial, "exponent()" for exponentiation, "gcd()" for GCD
// example code for below for how to operate this functions
BigInt ans = bigint_one.fact(bigint_second);
// cout<<ans.getInt() will display the results.
// Similary for others as well

--------------Thanks------------------