Date: 2026-06-06
Time: 18:49
remove-palindromic-subsequences/solution.pyThis file solves LeetCode 1332 — Remove Palindromic Subsequences. It determines the minimum number of steps to remove all characters from a string consisting only of 'a' and 'b', where each step removes a palindromic subsequence (not substring).
The key insight that makes this problem trivial: since the alphabet is only {a, b}, you can always remove all 'a's in one step (they form a palindromic subsequence) and all 'b's in another. So the answer is always 0, 1, or 2.
countStrings(s: str) -> int — The core solver. Contract:
'a' and 'b' characters0 if empty, 1 if the string is already a palindrome, 2 otherwiseThe function name countStrings is a misnomer — it counts *steps*, not strings. The LeetCode problem's canonical name is removePalindromeSub.
TestCountStrings — Unit test class covering edge cases: empty string, single char, palindromes, non-palindromes, uniform strings, alternating patterns, and a 1000-char stress test.
left and right converge toward the center. On first mismatch, short-circuit to return 2. This avoids creating a reversed copy (s == s[::-1]), saving O(n) space.python solution.py or a test runner. This is the repo-wide convention.Imports: Only unittest from the standard library — no external dependencies.
Imported by: The test_solution.py file in the same directory imports this. The massive "Imported By" list in the context is noise — those are *other* problems' test files that happen to share the same unittest import, not actual consumers of countStrings.
1. Empty check → return 0
2. Initialize two pointers at string boundaries
3. Walk inward comparing s[left] vs s[right]
4. First mismatch → return 2 (need two steps: one for all as, one for all bs)
5. Pointers meet without mismatch → return 1 (string is a palindrome, remove it whole)
{0, 1, 2} — this holds because the alphabet has exactly two characters. With 3+ characters, the problem would be fundamentally different.'a' and 'b'. It doesn't validate this, but the logic is correct regardless — for any binary-alphabet string, the max answer is 2.None. The function handles the empty-string edge case explicitly but has no error paths. Invalid input (non-ab strings) won't crash — the function just checks palindromicity, which is alphabet-agnostic. The answer of 2 would be wrong for strings with 3+ distinct characters, but the problem constrains this.
remove-palindromic-subsequences/plan.md — How the solution strategy was derived before implementationremove-palindromic-subsequences/review.md — Post-implementation review notes and quality assessmentsubsequence-vs-substring — Understanding why "palindromic subsequence" makes this problem trivially bounded, unlike "palindromic substring"valid-palindrome/solution.py:isPalindrome — Compare the two-pointer palindrome technique with the version that handles non-alphanumeric characterslongest-palindrome/solution.py — A related palindrome problem with a fundamentally different approachremove-palindrome-max-answer-is-two — For any non-empty string over a 2-character alphabet, at most 2 palindromic subsequence removals are needed (one per character)countstrings-is-pure — countStrings is a pure function with no side effects, no mutation, and no external statetwo-pointer-avoids-reversal-copy — The implementation uses O(1) space by walking pointers inward rather than comparing s == s[::-1] which allocates an O(n) reversed copyfunction-name-mismatches-leetcode — The function is named countStrings but the LeetCode problem expects removePalindromeSub; this is a naming inconsistency in the repo