Recursive function to generate binary tree

Question

I'm trying to create a function "generate_tree(L)", that takes a list of letters and returns a binary tree. Where each letter is a leaf. for example, with the list:

L = ['A', 'B', 'C']

the function should randomly return one of these trees:

['A',['B', 'C']] or [['A','B'], 'C']

Where the first example represent this tree:

binary tree

Well it's not going too well - I had a solution but it also returned a lot of empty lists and it was a mess. This is my attempt so far:

from random import randint
def generate_tree(L):
    n = len(L)
    if not L: #base case yield empty
        yield L
        return
    randomSplit = randint(1,n-1)
    leftList = L[0:randomSplit]
    rightList = L[randomSplit:]

List = ['A','B','C']
list(generate_tree(L))

My attempt with randomSplit is to divide the list into two lists in a random non-empty position:

['A','B','C'] --> leftList: ['A'], rightList: ['B','C']

I know I need to send leftList and rightList recursively, but I can't get it right. I would love your take on this issue

quamrana · Accepted Answer · 2019-10-30 10:42:06Z

1

This solution chooses a random split point and divides the list down from there:

from random import randint

def tree(lst):
    if len(lst) == 1:
        return lst[0]
    elif len(lst) <= 2:
        return lst
    else:
        ix = randint(1, len(lst) - 1)
        retval = [tree(lst[:ix]) , tree(lst[ix:])]
        return retval

tree(['A','B','C']) only produces one of the two required outputs.

Sample outputs from:

for _ in range(10):
    print(tree(['A','B','C','D']))

[['A', 'B'], ['C', 'D']]
[[['A', 'B'], 'C'], 'D']
['A', [['B', 'C'], 'D']]
[['A', 'B'], ['C', 'D']]
[['A', 'B'], ['C', 'D']]
['A', [['B', 'C'], 'D']]
[['A', ['B', 'C']], 'D']
[['A', ['B', 'C']], 'D']
[[['A', 'B'], 'C'], 'D']
['A', ['B', ['C', 'D']]]

answered Oct 30, 2019 at 10:42

quamrana

39.5k13 gold badges57 silver badges77 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

gstukelj · Accepted Answer · 2019-10-30 11:03:51Z

1

I think your base case is when length of list is 2 (or smaller in special cases), so here's an idea based on using random.randint:

from random import randint

def tree(lst):
    if len(lst) <= 2:
        return lst
    else:
        ix = randint(0, len(lst) - 1)
        retval = [lst[ix]]
        retval.append(tree(lst[:ix] + lst[ix+1:]))
        return retval

It should work also for list of length greater than 3 or of length 0 or 1.

EDIT:

If the first element of the list must be the first element (as suggested by your example), this should do:

from random import randint

def tree(lst):
    if len(lst) == 1:
        return lst[0]
    if len(lst) <= 2: # this matters is lst == []
        return lst
    else:
        ix = randint(1, len(lst) - 1)
        retval = [lst[:ix]] if ix > 1 else lst[:ix]
        retval.append(tree(lst[ix:]))
        return retval

edited Oct 30, 2019 at 11:03

answered Oct 30, 2019 at 10:10

gstukelj

2,5591 gold badge11 silver badges21 bronze badges

1 Comment

quamrana Over a year ago

For the case of ['A', 'B', 'C'] your function possibly produces additional results beyond the two alternatives that the OP requires.

Collectives™ on Stack Overflow

Recursive function to generate binary tree

2 Answers 2

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related