def solve(s):
    palindromes = [['' for i in range(len(s))] for j in range(len(s))]
    n = len(s) - 1
    result = solve_memoized_rec(s, palindromes, 0, n)
    return result
def solve_memoized_rec(s, palindromes, i, j):
    if palindromes[i][j]:
        return palindromes[i][j]
    else:
        if i > j:
            solution = ''
        elif i == j:
            solution = s[i]
        elif s[i] == s[j]:
            prev = solve_memoized_rec(s, palindromes, i + 1, j - 1)
            solution = s[i] + prev + s[j]
        else:
            from_left = solve_memoized_rec(s, palindromes, i + 1, j)
            from_right = solve_memoized_rec(s, palindromes, i, j - 1)
            both = solve_memoized_rec(s, palindromes, i + 1, j - 1)
            solution = max(from_left, from_right, both, key=len)
        palindromes[i][j] = solution
        return solution

def solve_iterative(s):
    n = len(s)
    palindromes = [['' for i in range(n)] for j in range(n)]
    for k in range(1, n + 1):
        for i in range(n - k + 1):
            j = i + k - 1
            if i == j:
                solution = s[i]
            elif s[i] == s[j]:
                solution = s[i] + palindromes[i + 1][j - 1] + s[j]
            else:
                solution = max(palindromes[i + 1][j], palindromes[i][j - 1], palindromes[i + 1][j - 1], key=len)
            palindromes[i][j] = solution
    return palindromes[0][n - 1]