Longest word chain from a list of words

You can use recursion to explore every “branch” that emerges when every possible letter containing the proper initial character is added to a running list:

words = ['giraffe', 'elephant', 'ant', 'tiger', 'racoon', 'cat', 'hedgehog', 'mouse']
def get_results(_start, _current, _seen):
  if all(c in _seen for c in words if c[0] == _start[-1]):
    yield _current
  else:
      for i in words:
        if i[0] == _start[-1]:
          yield from get_results(i, _current+[i], _seen+[i])


new_d = [list(get_results(i, [i], []))[0] for i in words]
final_d = max([i for i in new_d if len(i) == len(set(i))], key=len)

Output:

['hedgehog', 'giraffe', 'elephant', 'tiger', 'racoon']

This solution works similar to the breadth-first search, as the function get_resuls will continue to iterate over the entire list as long as the current value has not been called on before. Values that have been seen by the function are added to the _seen list, ultimately ceasing the stream of recursive calls.

This solution will also ignore results with duplicates:

words = ['giraffe', 'elephant', 'ant', 'ning', 'tiger', 'racoon', 'cat', 'hedgehog', 'mouse',]
new_d = [list(get_results(i, [i], []))[0] for i in words]
final_d = max([i for i in new_d if len(i) == len(set(i))], key=len)

Output:

['ant', 'tiger', 'racoon', 'ning', 'giraffe', 'elephant']