input: Write a function to find the number of valid parentheses combinations.
  assert find_valid_parens_combinations(0) == 0
  assert find_valid_parens_combinations(1) == 1
  assert find_valid_parens_combinations(2) == 1
  assert find_valid_parens_combinations(3) == 2
  assert find_valid_parens_combinations(4) == 1
output: def find_valid_parens_combinations(n):
      if n == 0:
          return 0
      def generate_parenthesis(p, left, right, parens=[]):
          if left:         generate_parenthesis(p + '(', left-1, right)
          if right > left: generate_parenthesis(p + ')', left, right-1)
          if not right:    parens += p,
          return parens
      return len(generate_parenthesis('', n, n))