erlang: cracking the coding interview 8.1

erlang/steps.erl

@@ -0,0 +1,14 @@
+-module(steps).
+-export([permutations/1, combinations/1]).
+
+% Cracking the Coding Interview 8.1
+% Go up N steps 1, 2, or 3 at a time
+
+% Count of permutations
+permutations(0) -> 0;
+permutations(1) -> 1;
+permutations(2) -> 2;
+permutations(Steps) -> permutations(Steps - 1) + permutations(Steps - 2) + permutations(Steps - 3).
+
+% Brute force list of all combinations O(N^3)
+combinations(N) -> [{X, Y, Z} || X <- lists:seq(0, N), Y <- lists:seq(0, N), Z <- lists:seq(0, N), (X + Y * 2 + Z * 3 == N)].