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"Implement a function that finds the longest palindromic substring in a given string."

def longestPalindrome(s: str) -> str:
    if len(s) < 2:
        return s
    
    start, max_len = 0, 1
    
    def expand(l, r):
        while l >= 0 and r < len(s) and s[l] == s[r]:
            l -= 1
            r += 1
        return l + 1, r - l - 1
    
    for i in range(len(s)):
        # Odd length
        l1, len1 = expand(i, i)
        if len1 > max_len:
            start, max_len = l1, len1
        
        # Even length  
        l2, len2 = expand(i, i + 1)
        if len2 > max_len:
            start, max_len = l2, len2
    
    return s[start:start + max_len]