void setup() {
  Serial.begin(9600); // initialize serial communication
}

void loop() {
  int n = 100; // number to calculate factorial of
  const int MAX_DIGITS = 200; // maximum number of digits to store
  int digits[MAX_DIGITS]; // array to store digits of result
  int num_digits = 1; // variable to store number of digits in result
  digits[0] = 1; // set first digit to 1
  
  // calculate factorial
  for(int i=2; i<=n; i++){
    // multiply each digit by i
    int carry = 0;
    for(int j=0; j<num_digits; j++){
      int product = digits[j] * i + carry;
      digits[j] = product % 10;
      carry = product / 10;
    }
    
    // add any remaining carry to new digits
    while(carry > 0){
      digits[num_digits] = carry % 10;
      carry /= 10;
      num_digits++;
    }
  }
  
  // print result to serial monitor
  Serial.print("100! = ");
  for(int i=num_digits-1; i>=0; i--){
    Serial.print(digits[i]);
  }
  Serial.println();
  
  while(1); // loop indefinitely
}