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Fix/palindrome separator collision #2772

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Merged
keon merged 2 commits into from
Sep 18, 2026
Merged

Fix/palindrome separator collision #2772

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c760a1b
fix(string): preserve # characters in longest palindrome
Miladkhoshdel Sep 17, 2026
55a62f5
test(string): cover palindromes containing #
Miladkhoshdel Sep 17, 2026
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12 changes: 6 additions & 6 deletions algorithms/string/longest_palindromic_substring.py
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Original file line number Diff line number Diff line change
Expand Up @@ -30,7 +30,10 @@ def longest_palindrome(text: str) -> str:
if len(text) < 2:
return text

expanded = "#" + "#".join(text) + "#"
separator = object()
expanded: list[object] = [separator]
for char in text:
expanded.extend((char, separator))
palindrome_radii = [0] * len(expanded)
center, right_boundary = 0, 0
best_index, best_length = 0, 0
Expand Down Expand Up @@ -60,8 +63,5 @@ def longest_palindrome(text: str) -> str:
best_index = index
best_length = palindrome_radii[index]

substring = expanded[
best_index - palindrome_radii[best_index] + 1 : best_index
+ palindrome_radii[best_index]
]
return substring.replace("#", "")
start = (best_index - best_length + 1) // 2
return text[start : start + best_length - 1]
2 changes: 2 additions & 0 deletions tests/test_string.py
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Expand Up @@ -636,6 +636,8 @@ def test_longest_palindromic_substring(self):
self.assertEqual("abba", longest_palindrome("abba"))
self.assertEqual("asdadsa", longest_palindrome("dasdasdasdasdasdadsa"))
self.assertEqual("abba", longest_palindrome("cabba"))
self.assertEqual("a#a", longest_palindrome("a#a"))
self.assertEqual("##", longest_palindrome("##"))


class TestKnuthMorrisPratt(unittest.TestCase):
Expand Down