Permutation and Combination in Java

[Sandeep Reddy Vinta]

HI,
Is there any utility class in java which helps in computing Permutation and Combination?

[Paul Clapham]

The calculations themselves are very simple but you might require BigInteger if your numbers are going to become large.

[Campbell Ritchie]

Go to the API index (that was for P) and try ctrl-F permutation. And you will probably find the same as I did . . . nothing. You will have to try with C for combination, too.

[Campbell Ritchie]

Combination =
       n!       
r! × (n - r)!

Permutation =
   n!  
   r!

In all cases n ≥ r and if n = r both nCr = 1 and nPr = 1

You can probably count down from n and up from r; if you multiply downwards, you can reduce the maximum size of intermediate values. Don’t try to work out 10 × 9 × 8 × 7 × 6 × 5 × 4 × 3 × 2 × 1 ÷ 7 ÷ 6 ÷ 5 ÷ 4 ÷ 3 ÷ 2 ÷ 1.
Try 10 ÷ 1 × 9 ÷ 2 × 8 ÷ 3 × 7 ÷ 4 × 6 ÷ 5 × 5 ÷ 6 × 4 ÷ 7 × 3 × 2 × 1
You can speed the process, at the cost of more complicated code, if you can work out how to remove common factors and divisors.