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Description

Updated the Factorial.java implementation to use BigInteger instead of long.

Reason for change:
The previous implementation using long would overflow for any input $n > 20$. Since factorials grow exponentially, BigInteger is necessary to handle arbitrarily large results accurately without data loss.

Changes:

• Changed return type to BigInteger.
• Updated the iterative loop to use BigInteger.multiply().
• Added a check for negative input to throw IllegalArgumentException.