When the order doesn't matter, it is a Combination.
When the order does matter it is a Permutation.
"The combination to the safe is 472". We do care about the order. 724 won't work, nor will 247.
It has to be exactly 4-7-2.
A permutation, also called an “arrangement number” or “order”, is a rearrangement of
the elements of an ordered list S into a one-to-one correspondence with S itself.
Below are the permutations of string ABC.
ABC ACB BAC BCA CBA CAB
Or for example the first three people in a running race: you can't be first and second.
Number of combinations
n * (n-1) * (n -2) * ... * 1 = n!
When repetition is allowed we have permutations with repetitions.
For example the the lock below: it could be 333.
Number of combinations
n * n * n ... (r times) = n^r
Permutations cheat sheet
Combinations cheat sheet
Permutations/combinations algorithm ideas.
