A more elegant way to return a list of node values from depth-first recursive function in Python 3?

Question

I’m working out how to return a list of values from a depth-first traversal of a binary tree. Specifically, I’m trying to do so with a recursive function.

After playing around a bit I got this code to properly return the list of expected values, but using a try/finally block in order to pull it off seems clunky to me.

def depth_first_recursive(root, node_list: list):
    try:
        if not root:
            return
        else:
            node_list.append(root.val)
            depth_first_recursive(root.left, node_list)
            depth_first_recursive(root.right, node_list)
    finally:
        return node_list


class Node:
    def __init__(self, val):
        self.val = val
        self.left = None
        self.right = None
        values = []


if __name__ == '__main__':
    a = Node('a')
    b = Node('b')
    c = Node('c')
    d = Node('d')
    e = Node('e')
    f = Node('f')

    a.left = b
    a.right = c
    b.left = d
    b.right = e
    c.right = f

    nodelist = []
    print(depth_first_recursive(a, node_list=nodelist))

returns ['a', 'b', 'd', 'e', 'c', 'f', ]

Is there a better way to pull this off in Python?

Asked By: Sirrah

||

Source

Answer 1

I have applied the same logic as yours but tried to make code compact:

def depth_first_recursive(root, node_list: list):
    if root is None: return []
    return [root.val]+depth_first_recursive(root.left, node_list) + depth_first_recursive(root.right, node_list)

Output:

[‘a’, ‘b’, ‘d’, ‘e’, ‘c’, ‘f’]

Answered By: Sakshi Sharma

Answer 2

Don’t need to maintain a node_list, just have more faith in the structure of your recursion.

def depth_first_recursive(root):

    if root is None:
        return []
    # Terminal case when reaching bottom of tree
    elif not root.left and not root.right:
        return [root.val]
    # Get current value and continue to append list
    else:
        return [root.val] + depth_first_recursive(root.left) + depth_first_recursive(root.right)

Answered By: Michael Cao

A more elegant way to return a list of node values from depth-first recursive function in Python 3?

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