How to this python code for max depth using DFS working here?

Question

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

def max_depth(root):
    return root and 1 + max(max_depth(root.left), max_depth(root.right)) or 0

if __name__ =="__main__":
    def build_tree(nodes):
        val = next(nodes)
        if not val or val == 'x': return 
        cur = Node(int(val))
        cur.left = build_tree(nodes)
        cur.right = build_tree(nodes)
        return cur
    root = build_tree(iter(input().split()))
    print(max_depth(root))

Here it's giving the correct answer. But i can't understand how the max_depth function working here. More specifically the 'and' and 'or' operation here.

What do you understand about it?

Scott Hunter
– Scott Hunter

2020-10-08 18:18:58 +00:00
Commented Oct 8, 2020 at 18:18 — Scott Hunter
– Scott Hunter, Commented Oct 8, 2020 at 18:18

trincot · Accepted Answer · 2020-10-08 18:25:34Z

2

This expression:

root and 1 + max(max_depth(root.left), max_depth(root.right)) or 0

is equivalent to:

1 + max(max_depth(root.left), max_depth(root.right)) if root else 0

This is because and will return the first operand when it is falsy, and the second operand if the first is truthy. The or operand will return its first operand when it is truthy, and its second operand when the first is falsy.

You can also write it more verbose with separate return statements:

if root:
    return 1 + max(max_depth(root.left), max_depth(root.right))
else:
    return 0

answered Oct 8, 2020 at 18:25

trincot

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1 Comment

Jorge Morgado Over a year ago

This is definitely a much better answer. Nice :)

Jorge Morgado · Accepted Answer · 2020-10-08 18:19:51Z

1

The and is used to check if the root is not None. So it is like doing:

0 if root is None else 1 + max(max_depth(root.left), max_depth(root.right))

answered Oct 8, 2020 at 18:19

Jorge Morgado

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