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The data structure classes from leetcode_py convert between LeetCode’s array format and live objects, and render themselves so you can look at what your code is doing.

from leetcode_py import GraphNode, ListNode, TreeNode

Array format, live objects

root = TreeNode.from_list([3, 9, 20, None, None, 15, 7])
root.to_list()          # [3, 9, 20, None, None, 15, 7]

head = ListNode.from_list([1, 2, 3])
head.to_list()          # [1, 2, 3]

graph = GraphNode.from_adjacency_list([[2, 4], [1, 3], [2, 4], [1, 3]])
GraphNode.to_adjacency_list(graph)  # [[2, 4], [1, 3], [2, 4], [1, 3]]

Cyclic lists are safe: traversal detects cycles instead of hanging, in both directions. Each class carries a few extras beyond conversion and rendering:

TreeNode.find_node(value) returns the first node with that value.
GraphNode.is_clone(other) checks deep equality between a graph and its copy, the shape clone-graph problems ask for.
All three are generic: TreeNode[int], ListNode[str], and so on.
from leetcode_py.data_structures import DictTree gives a dict-backed tree that renders with box-drawing characters in the terminal and Graphviz in Jupyter, which suits Trie implementations.

In Jupyter: diagrams

As the last expression of a cell, these objects render as Graphviz SVG diagrams. TreeNode lays out top-to-bottom; ListNode goes left-to-right with rounded boxes. A cyclic list draws the back-edge in red, dashed, and labeled cycle.

head = ListNode.from_list([1, 2, 3])
head.next.next.next = head.next   # cycle: 3 -> 2
head                              # renders the diagram, cycle included

TreeNode rendered as a Graphviz tree diagram in Jupyter

ListNode rendered as a left-to-right box diagram in Jupyter

In the terminal: ASCII

print() falls back to clean text: an indented tree, an arrow chain for lists (cycles shown as ... (cycle back to 2)), and a formatted adjacency dict for graphs.

print(root)
# 3
# ├── 9
# └── 20
#     ├── 15
#     └── 7

TreeNode printed as an ASCII tree in the terminal

ListNode printed as an arrow chain in the terminal

Requirements

Diagram rendering needs the system Graphviz binary (see Installation). If it is missing, the objects fall back to the ASCII form, so you get readable output either way.

Last modified on August 24, 2026