C program

Write a recursive function that displays all the binary (base 2) numbers represented by a string of x s, 0 s, and 1 s. The x s represent digits that can be either 0 or 1 . For example, the string 1x0x represents the numbers 1000 , 1001 , 1100 , 1101 . The string xx1 represents 001 , 011 , 101 , 111 . Hint: Write a helper function replace_first_x that builds two strings based on its input argument. In one, the first x is replaced by a 0 , and in the other by a 1 . The set function is_element may be useful too.