Notebook

589. N-ary Tree Preorder Traversal

Given an n-ary tree, return the preorder traversal of its nodes' values.

Nary-Tree input serialization is represented in their level order traversal, each group of children is separated by the null value (See examples).

Follow up:

Recursive solution is trivial, could you do it iteratively?

Example 1:

Input: root = [1,null,3,2,4,null,5,6]
Output: [1,3,5,6,2,4]

Example 2:

Input: root = [1,null,2,3,4,5,null,null,6,7,null,8,null,9,10,null,null,11,null,12,null,13,null,null,14]
Output: [1,2,3,6,7,11,14,4,8,12,5,9,13,10]

Constraints:

The height of the n-ary tree is less than or equal to 1000
The total number of nodes is between [0, 10^4]

Source

In [3]:

# Definition for a Node.
class Node:
    def __init__(self, val=None, children=None):
        self.val = val
        self.children = children  # children = list

Code

In [4]:

def preorder(root):
    """DFS using recursion."""
    if not root:
        return []
    traversal = [root.val]  # would come last in postorder
    for child in root.children:
        traversal.extend(preorder(child))
    # !! traversal.extend(preorder(child) for child in root.children) does NOT work
    # leads to --> traversal = [w, [x, [y, [z]]]]
    return traversal

In [1]:

def preorder(root):
    """alternative version of above solution."""
    def helper(node):
        if node is None:
            return None
        nonlocal result
        traversal.append(node.val)  # would come last in postorder
        for child in node.children:
            helper(child)

    traversal = []
    helper(root)
    return traversal

Follow up:

Solve it both recursively and iteratively.

Code

In [ ]:

def preorder(root):
    """DFS using an iterative approach with a stack."""
    if not root:
        return []
    stack = [root]
    traversal = []
    while stack:
        node = stack.pop()
        traversal.append(node.val)
        stack += reversed(node.children)  # reversed() to ensure left subtree is processed first
        # stack.append(reversed(node.children)) doesnt work because reversed() is an iterator
        # Instead, we can do:
        #     stack.extend(reversed(node.children))
        # OR
        #     for child in reversed(node.children):
        #         stack.append(child)
    return traversal