def longest_common_subsequence(first, second):
    if not isinstance(first, str) or not isinstance(second, str):
        raise ValueError("String inputs required")
    if (len(first) + 1) * (len(second) + 1) > 100000:
        raise ValueError("Teaching table budget exceeded")
    table = [[0] * (len(second) + 1) for _ in range(len(first) + 1)]
    for row in range(1, len(first) + 1):
        for column in range(1, len(second) + 1):
            if first[row - 1] == second[column - 1]:
                table[row][column] = table[row - 1][column - 1] + 1
            else:
                table[row][column] = max(table[row - 1][column], table[row][column - 1])
    row, column, result = len(first), len(second), []
    while row and column:
        if first[row - 1] == second[column - 1]:
            result.append(first[row - 1])
            row -= 1
            column -= 1
        elif table[row - 1][column] >= table[row][column - 1]:
            row -= 1
        else:
            column -= 1
    return "".join(reversed(result))

result = longest_common_subsequence("orchard", "chart")
print(result, len(result))
print(repr(longest_common_subsequence("", "oak")))