dynamic-programming(i): add day 1

Signed-off-by: Matej Focko <mfocko@redhat.com>

study-plan/dynamic-programming/day-1/fibonacci-number.rb

@@ -0,0 +1,39 @@
+# @param {Integer} n
+# @return {Integer}
+def fib(n)
+    if n == 0 then
+        return 0
+    end
+
+    prev, current = 0, 1
+    (2..n).each {
+        prev, current = current, prev + current
+    }
+    return current
+end
+
+RSpec.describe "fib of " do
+    it "0 is 0" do
+        expect(fib(0)).to eq(0)
+    end
+
+    it "1 is 1" do
+        expect(fib(1)).to eq(1)
+    end
+
+    it "2 is 1" do
+        expect(fib(2)).to eq(1)
+    end
+
+    it "3 is 2" do
+        expect(fib(3)).to eq(2)
+    end
+
+    it "4 is 3" do
+        expect(fib(4)).to eq(3)
+    end
+
+    it "5 is 5" do
+        expect(fib(5)).to eq(5)
+    end
+end

study-plan/dynamic-programming/day-1/tribonacci.rb

@@ -0,0 +1,43 @@
+# @param {Integer} n
+# @return {Integer}
+def tribonacci(n)
+    sequence = [0, 1, 1]
+    if n < 3 then
+        return sequence[n]
+    end
+
+    (3..n).each {
+        sequence = sequence.drop(1) + [sequence.sum]
+    }
+    return sequence.last
+end
+
+RSpec.describe "tribonacci of " do
+    it "0 is 0" do
+        expect(tribonacci(0)).to eq(0)
+    end
+
+    it "1 is 1" do
+        expect(tribonacci(1)).to eq(1)
+    end
+
+    it "2 is 1" do
+        expect(tribonacci(2)).to eq(1)
+    end
+
+    it "3 is 2" do
+        expect(tribonacci(3)).to eq(2)
+    end
+
+    it "4 is 4" do
+        expect(tribonacci(4)).to eq(4)
+    end
+
+    it "5 is 7" do
+        expect(tribonacci(5)).to eq(7)
+    end
+
+    it "25 is 1389537" do
+        expect(tribonacci(25)).to eq(1389537)
+    end
+end