Date: 2026-06-06
Time: 18:57
This file is the self-contained solution for LeetCode 2236: Root Equals Sum of Children. It owns three things: the TreeNode data structure, the Solution.checkTree algorithm, and the test suite that validates it. It's one of the simplest problems in the repository — a guaranteed 3-node binary tree where you check if root.val == left.val + right.val.
TreeNode (lines 9–14)Standard binary tree node with val, left, and right. This is the local definition used by both the solution and tests — it mirrors LeetCode's provided definition rather than importing a shared one.
Solution.checkTree (lines 18–26)Single-expression method: return root.val == root.left.val + root.right.val. No null-checking, no recursion, no edge-case handling — the problem guarantees exactly 3 nodes with both children present.
TestCheckTree (lines 29–48)Seven test cases covering:
The helper maketree constructs the fixed 3-node tree from three ints, keeping test setup DRY.
TreeNode is defined locally rather than imported from a shared utility, which is the convention across this repository. Every problem directory is independent.Solution class method, even though a bare function would suffice.testsolution.py in the directory likely imports from here). The if name == "main_" guard lets the file run standalone.Imports: annotations (PEP 604-style type hints), unittest, Optional from typing. All stdlib — no external packages.
Imported by: The "Imported By" list is enormous (~400+ test files), which almost certainly means those test files import TreeNode from this module as a shared fixture — not Solution or checkTree. This file is effectively the canonical TreeNode definition for the entire repository's test infrastructure.
1. Caller passes a TreeNode root guaranteed to have both .left and .right.
2. checkTree reads three .val attributes, computes one addition, one comparison.
3. Returns a bool. O(1) time, O(1) space.
root.left.val and root.right.val unconditionally — any violation of this precondition raises AttributeError.None. The method trusts the caller to satisfy the precondition (3-node tree). A None root or missing child causes an unhandled AttributeError. This is appropriate given LeetCode's guarantees.
evaluate-boolean-binary-tree/solution.py — Another simple binary tree problem; compare how it handles recursive evaluation vs. this flat checkbalanced-binary-tree/solution.py — Shows recursive tree traversal with height computation, a step up in complexity from this single-node checkTreeNode-reuse-across-tests — Investigate why ~400 test files import from this module; is it intentional or an artifact of the test generation script?root-equals-sum-of-children/test_solution.py:TestCheckTree — Check whether the separate test file duplicates or extends the inline testsrun_tests.py — The project-wide test runner; understanding how it discovers and runs these per-problem test suitesroot-equals-sum-no-null-guard — checkTree unconditionally dereferences root.left and root.right with no null checks, relying on the problem's 3-node guaranteetreenode-is-canonical-import — This file's TreeNode class is imported by 400+ test files across the repository, making it the de facto shared tree node definitionsolution-is-o1 — checkTree performs exactly one addition and one comparison with no recursion or iteration, making it O(1) time and spacetests-cover-negative-values — The test suite validates negative and mixed-sign node values, which goes beyond LeetCode's stated constraint of non-negative values