Struggling with PairInverse problem in c using recursion [duplicate] - c

Question

I encountered a hard question I don't know the answer to: "Rearrange the digits from an integer in blocks of two with a recursive function" here's an example:
Input: 123456
unsigned long pairinvPrint(unsigned long number) {
printf("%d", number % 100);
if ((number / 100) <= 99) {
printf("%d", number / 100);
}
else {
pairinv(number / 100);
}
}
Output: 563412
More I/O Examples: 42 -> 42; 1234 -> 3412
However, the set circumstances to do this are hard (no loops, arrays, pointers, global- or static variables, no libraries) and it should not print the solution directly, rather return it upon a call like this:
printf("Rearrange int (%lu) = %lu", input, pairinvert(input));
Luckily there's one circumstance to make it easier, the number of the input digits is always even.
Now I experimented for a while, but cant come up with a working solution, except the invalid one using printf.
Does anyone have some inspiration for me or idea how to tackle this?

Answer 1

I'll bite :-)
unsigned long p(unsigned long p1, unsigned long p2) {
// no loops, no arrays, no pointers, no global, no static, no variables, no libraries
if (p1 < 100) return p2*100 + p1;
return p(p1/100, p2*100 + p1%100);
}
unsigned long pairinvert(unsigned long n) {
// no loops, no arrays, no pointers, no global, no static, no variables, no libraries
if (n < 100) return n;
return p(n/100, n%100);
}
// need <stdio.h> for printf()
#include <stdio.h>
int main(void) {
unsigned long input;
input = 123456;
printf("Rearrange int (%lu) = %lu\n", input, pairinvert(input));
input = 42;
printf("Rearrange int (%lu) = %lu\n", input, pairinvert(input));
input = 1234;
printf("Rearrange int (%lu) = %lu\n", input, pairinvert(input));
}

Answer 2

Following program should work.
#include <stdio.h>
void rearrange(int n, int *output) {
int lsd = 0, slsd = 0;
if(n == 0)
return;
if(n > 0) {
lsd = n%10;
}
if (n > 9) {
slsd = (n%100)/10;
}
*output = 100*(*output) + 10*slsd + lsd;
n = n/100;
rearrange(n, output);
}
int main() {
int n;
int output = 0;
scanf("%d", &n);
rearrange(n, &output);
printf("%d\n", output);
return 0;
}
It is simple to understand, so I am not writing any comments.
Note that it is tail recursive so with O2 optimization it can recurse infinitely.

Answer 3

Try this :
unsigned long pairinv(unsigned long number, unsigned long result) {
unsigned long n = number % 100; // Gets the two digit number
if (n == 0) return result; // If it's zero returns the result
result = result * 100 + n; // Else multiplies the result by 100, adds n
return pairinv(number / 100, result); // and continues by recursion
}
int main() {
unsigned long r= 0;
printf("%lu\n", pairinv(123456, r)); //==> 563412
return 0;
}