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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def check_int(integer):
 """ Check if number is integer or not. :param integer: Number as str :return: Boolean """

    if not isinstance(integer, str):
        return False
    if integer[0] in ('-', '+'):
        return integer[1:].isdigit()
    return integer.isdigit()