#Recursion Practices with TDD Including common recursion problems (implementation in Ruby):

1. range

range(5, 10) 
#=> [6, 7, 8, 9]

2. sum

[1, 2, 3].sum
#=> 6

3. exponent

exp(2, 9)
#=> 512
exp(2, 7)
#=> 128

4. Deep dup that handles arrays inside an array

test_arr = [[0, 1], [2, 3]]
test_arr.deep_dup
#=> [[0, 1], [2, 3]]
test_arr.object_id == test_arr.deep_dup.object_id
#=> false

5. Fibonacci Sequence

fibs(6)
#=> [0, 1, 1, 2, 3, 5]

6. Make Change

make_change(14)
#=> [10, 1, 1, 1, 1]

7. Binary Search

bsearch([0, 1, 2, 3, 4, 5], 3)
#=> 3
bsearch([0, 1, 2, 3, 4, 5], 0)
#=> 0
bsearch([0, 1, 2, 3, 4, 5], 5)
#=> 5
bsearch([0, 1, 2, 3, 4, 5], 6)
#=> nil

8. Merge Sort

merge_sort([2, 3, 1, 5, 2, 8, 9])
#=> [1, 2, 2, 3, 5, 8, 9]
# merge_sort runs in O (n log n) time

9. Subsets

subsets([1, 2, 3])
#=> [[], [1], [2], [1, 2], [3], [1, 3], [2, 3], [1, 2, 3]]