给定程序中，函数fun的功能是：找出100~999之间（含100和999）所有整数中各位上数字之和为x(x为一正整数）的整数，
然后输出；符合条件的整数个数作为函数值返回*****/(Given a program, the function fun function is: to find between 100~999 (including 100 and 999), all integers in which the number of integers and X (x is a positive integer) integer,
Then the output; integer number of qualified as a function return value *****/)