def int_to_base36(integer: int) -> str:
    """Convert an integer to a base36 string."""
    char_set = "0123456789abcdefghijklmnopqrstuvwxyz"
    if integer < 0:
        raise ValueError("Negative base36 conversion input.")
    if integer < 36:
        return char_set[integer]
    b36 = ""
    while integer != 0:
        integer, index = divmod(integer, 36)
        b36 = char_set[index] + b36
    return b36