feat(algorithms, dynamic programming): dynamic programming problems #151

BrianLusina · 2026-01-13T11:34:33Z

Describe your change:

Count number of palindromic substrings in a string

Add an algorithm?
Fix a bug or typo in an existing algorithm?
Documentation change?

Checklist:

Summary by CodeRabbit

Documentation
- Added comprehensive guides for palindromic substring problems, including problem definitions, solution approaches, and complexity analysis.
- Relocated palindromic substring content within the algorithms directory structure.
Tests
- Introduced test suites for palindromic substring algorithms to verify correctness across various input scenarios.
Style
- Updated documentation formatting for clarity.

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coderabbitai · 2026-01-13T11:34:49Z

📝 Walkthrough

Walkthrough

This change reorganizes palindromic substring problems from the two_pointers directory to the dynamic_programming directory. New implementations for counting palindromic substrings are added, along with comprehensive documentation and corresponding test modules. A minor docstring formatting update is also applied to pascals_triangle.

Changes

Cohort / File(s)	Summary
Documentation and Directory Updates `DIRECTORY.md`, `algorithms/dynamic_programming/palindromic_substring/README.md`, `algorithms/two_pointers/palindrome/README.md`	Added new Palindromic Substring and Pascals Triangle subsections to directory index under Dynamic Programming. Added comprehensive README documenting palindromic substring problems with DP-based solution outline, complexity analysis, and problem definitions. Removed Longest Palindromic Substring documentation from two_pointers section.
Palindromic Substring Core Implementations `algorithms/dynamic_programming/palindromic_substring/__init__.py`	Implements two DP-based functions: `count_palindromic_substrings` and `count_palindromic_substrings_2`, both counting all palindromic substrings in a given string using 2D DP tables with O(n²) time and space complexity.
Longest Palindromic Substring Function `algorithms/dynamic_programming/palindromic_substring/longest_palindromic_substring.py`	Updated import path for `is_palindrome` dependency from relative import to `algorithms.two_pointers.palindrome`. Adjusted docstring formatting by relocating content to plain string literal in function body.
Test Modules `algorithms/dynamic_programming/palindromic_substring/test_longest_palindromic_substring.py`, `algorithms/dynamic_programming/palindromic_substring/test_palindromic_substring.py`	Added parameterized unit tests for `longest_palindromic_substring` function and for both `count_palindromic_substrings` variants, covering simple, edge, and extended palindromic scenarios.
Minor Documentation Update `algorithms/dynamic_programming/pascals_triangle/__init__.py`	Corrected docstring formatting by changing ellipsis notation from "......," to "..," in sequence representation.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Poem

🐰 Our Dynamic codes now shine,
Palindromic substrings dance in line,
From two_pointers they did move,
To DP's recursive groove,
Tests and docs complete the rhyme! 🌟

🚥 Pre-merge checks | ❌ 3

❌ Failed checks (2 warnings, 1 inconclusive)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description is largely incomplete. While it marks multiple checklist items as complete, several key requirements appear unmet based on the actual changes made.	Update the description to accurately reflect all changes: multiple files modified (not just one algorithm), moved content between sections, and clarify what 'Fix a bug or typo' refers to since changes appear to be mostly additive and refactoring.
Docstring Coverage	⚠️ Warning	Docstring coverage is 57.14% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.
Title check	❓ Inconclusive	The title is vague and overly generic, using broad language like 'dynamic programming problems' without specifying the main change.	Revise the title to be more specific about the primary change, such as 'feat: Add palindromic substring counting algorithms' or similar.

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Actionable comments posted: 2

Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)

algorithms/dynamic_programming/palindromic_substring/longest_palindromic_substring.py (1)
32-32: Critical bug: i + i should be i + 1.

When setting the result bounds for a two-character palindrome, the end index calculation is incorrect. For phrase[i] == phrase[i + 1], the palindrome spans indices [i, i + 1], not [i, i + i].

Example: For input "aa", when i=0:

Current: result = [0, 0] → returns "a"

Expected: result = [0, 1] → returns "aa"
🐛 Proposed fix
         if phrase[i] == phrase[i + 1]:
             dp[i][i + 1] = True
-            result = [i, i + i]
+            result = [i, i + 1]

🤖 Fix all issues with AI agents

In @algorithms/dynamic_programming/palindromic_substring/README.md:
- Around line 68-69: Update the sentence that currently reads "lengths greater
than three" to "lengths greater than two" in the README explanation so it
correctly matches the algorithm description (base cases for lengths 1 and 2,
loop starts at length 3); ensure the surrounding text still reads naturally
after this single-word change.

In
@algorithms/dynamic_programming/palindromic_substring/test_longest_palindromic_substring.py:
- Around line 7-15: Add missing edge-case tests to
LONGEST_PALINDROMIC_SUBSTRING_TEST_CASES by including ("", ""), ("a", "a"), and
a length-2 palindrome case such as ("aab", "aa"); also fix the off-by-one bug in
longest_palindromic_substring.py where the center-expansion result is set with
result = [i, i + i] — change it to result = [i, i + 1] so two-character
palindromes are handled correctly (search for the variable name result and the
incorrect assignment to locate the fix).

🧹 Nitpick comments (1)

algorithms/dynamic_programming/palindromic_substring/__init__.py (1)
34-66: Consider documenting the difference between the two implementations.

Both functions produce identical results but use slightly different iteration patterns. The second function uses boolean arithmetic (count += dp[i][j]) which is a nice Pythonic touch.

It would be helpful to add a brief note in the docstring or a module-level comment explaining why two implementations exist (e.g., for educational purposes, different iteration order, etc.).
📝 Suggested docstring enhancement
 def count_palindromic_substrings_2(s: str) -> int:
     """
     Counts the number of palindromic substrings that can be found in the given string
+
+    This is an alternative implementation that iterates by substring length
+    rather than by difference between indices. Both approaches have the same
+    O(n²) time and space complexity.
+
     Args:
         s(str): string to check for palindromic substrings
     Returns:
         int: number of palindromic substrings
     """

📜 Review details

Configuration used: defaults

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 72a999a and 780f9dc.

⛔ Files ignored due to path filters (19)

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algorithms/dynamic_programming/palindromic_substring/images/examples/palindromic_substring_example_2.png is excluded by !**/*.png
algorithms/dynamic_programming/palindromic_substring/images/solutions/palindromic_substring_solution_1.png is excluded by !**/*.png
algorithms/dynamic_programming/palindromic_substring/images/solutions/palindromic_substring_solution_10.png is excluded by !**/*.png
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📒 Files selected for processing (10)

DIRECTORY.md
algorithms/dynamic_programming/palindromic_substring/README.md
algorithms/dynamic_programming/palindromic_substring/__init__.py
algorithms/dynamic_programming/palindromic_substring/longest_palindromic_substring.py
algorithms/dynamic_programming/palindromic_substring/test_longest_palindromic_substring.py
algorithms/dynamic_programming/palindromic_substring/test_palindromic_substring.py
algorithms/dynamic_programming/pascals_triangle/README.md
algorithms/dynamic_programming/pascals_triangle/__init__.py
algorithms/dynamic_programming/pascals_triangle/test_pascals_triangle.py
algorithms/two_pointers/palindrome/README.md

💤 Files with no reviewable changes (1)

algorithms/two_pointers/palindrome/README.md

🧰 Additional context used

🧬 Code graph analysis (1)

algorithms/dynamic_programming/palindromic_substring/test_longest_palindromic_substring.py (1)

algorithms/dynamic_programming/palindromic_substring/longest_palindromic_substring.py (1)

longest_palindromic_substring (4-42)

🪛 LanguageTool

algorithms/dynamic_programming/palindromic_substring/README.md

[style] ~23-~23: Consider a more expressive alternative.
Context: ...results of the earlier computations. To do this, we can create a lookup table, dp,...

(DO_ACHIEVE)

[style] ~43-~43: Using many exclamation marks might seem excessive (in this case: 15 exclamation marks for a text that’s 5567 characters long)
Context: ...s/palindromic_substring_solution_1.png) ![Solution 16](./images/solutions/palindr...

(EN_EXCESSIVE_EXCLAMATION)

🪛 markdownlint-cli2 (0.18.1)

algorithms/dynamic_programming/palindromic_substring/README.md

23-23: Reference links and images should use a label that is defined
Missing link or image reference definition: "j"