Write a method that returns the decimal equivalent of the hexadecimal number represented by the string 'hex'. As preconditions you may assume that 'hex' only contains the symbols: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, and F. You may also assume that 'hex', when converted to decimal, is less than the maximum possible integer value. If 'hex' is an empty string, return zero.