Date: 2026-06-06
Time: 19:39
This file implements LeetCode 125: Valid Palindrome. It determines whether a string reads the same forwards and backwards after stripping all non-alphanumeric characters and normalizing case. It's one of ~400+ solution files in the leetcode-implementations repo, each following the same Solution class convention that LeetCode expects.
Solution.isPalindrome(s: str) -> boolThe only method. Contract: given any string s, return True if it's a valid palindrome considering only alphanumeric characters and ignoring case, False otherwise.
Two-pointer inward sweep. Two indices start at opposite ends of the string and walk toward each other. Each pointer skips non-alphanumeric characters before comparing. This is the canonical O(n) time, O(1) space approach — no filtered copy of the string is ever allocated.
The alternative one-liner (sclean == sclean[::-1] after filtering) would be O(n) space. This solution avoids that.
Imports: None — uses only builtins (str.isalnum, str.lower).
Imported by: The "Imported By" list in the prompt is misleading — those are test files across the entire repo that share a common test harness importing from solution.py in their own directories, not files that import *this* solution. The only file that genuinely imports this module is valid-palindrome/test_solution.py.
1. Initialize left = 0, right = len(s) - 1.
2. Outer while left < right loop drives convergence.
3. Inner loops advance left forward and right backward past any non-alphanumeric characters (spaces, punctuation, etc.).
4. Compare s[left].lower() to s[right].lower(). On mismatch, return False immediately.
5. Move both pointers inward (left += 1, right -= 1).
6. If the loop completes without returning False, the string is a palindrome — return True.
For input "A man, a plan, a canal: Panama":
A/a, m/m, a/a, ...) matches case-insensitively.True.while loops re-check left < right, preventing out-of-bounds access on strings that are entirely non-alphanumeric (e.g., ",.!")..lower() is called at comparison time, not during preprocessing — the original string is never mutated or copied.left < right handles these correctly — they return True with zero comparisons.None. The method assumes s is a valid Python string (guaranteed by LeetCode's contract). No exceptions are raised or caught. Non-alphanumeric-only strings like "" or "!@#" are handled gracefully by the pointer guards and return True.
valid-palindrome/test_solution.py — See what edge cases are tested (empty string, single char, all non-alnum, mixed case)valid-palindrome-ii/solution.py — The follow-up problem: can you make it a palindrome by removing at most one character? Uses the same two-pointer foundation with a twistbackspace-string-compare/solution.py:backspaceCompare — Another two-pointer problem that walks strings from both ends, but with a different skip condition (backspace chars)palindrome-linked-list/solution.py — Same palindrome check applied to a linked list, where the O(1) space constraint forces a different structural approach (reverse half the list)valid-palindrome-uses-o1-space — isPalindrome uses O(1) auxiliary space; it never allocates a filtered or reversed copy of the input stringvalid-palindrome-inner-loop-guards — The inner skip loops re-check left < right, which prevents index-out-of-bounds on strings containing no alphanumeric charactersvalid-palindrome-case-insensitive-at-compare — Case normalization happens at comparison time via .lower(), not by preprocessing the entire stringvalid-palindrome-empty-is-palindrome — An empty string or a string with no alphanumeric characters returns True (the loop body never executes)