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How do you find the longest palindromic substring?

Q: How do you find the longest palindromic substring?

**Intuition.** Every palindrome has a center. There are 2n−1 possible centers (each character, and each gap between characters). **Expand around each center** outward while the two sides match, and remember the longest palindrome you grow. "babad": expand around center 'a' → "bab" b a b a <text x="34

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Expand around center

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def longest_palindrome(s):
    if not s: return ""
    start, end = 0, 0
    def expand(l, r):
        while l >= 0 and r < len(s) and s[l] == s[r]:
            l -= 1; r += 1
        return l + 1, r - 1
    for i in range(len(s)):
        for l, r in (expand(i, i), expand(i, i + 1)):
            if r - l > end - start:
                start, end = l, r
    return s[start:end + 1]


# --- demo ---
print(longest_palindrome("babad"))   # bab (or aba)
print(longest_palindrome("cbbd"))    # bb

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