Answer 1

OUTPUT:

RECURSION STARTED!

+ 1 PUSHED TO STACK (A) TOP +
RECURSIVE CALL FROM STACK (A)
+ 2 PUSHED TO STACK (A) TOP +
RECURSIVE CALL FROM STACK (A)
+ 3 PUSHED TO STACK (A) TOP +
RECURSIVE CALL FROM STACK (A)
>> \0 triggered BY STACK (A)!
! STACK (A) MIRRORED 3 TO STACK (B) !
RECURSIVE CALL FROM STACK (B)
>> \0 triggered BY STACK (B)!
! STACK (B) POPPED 3 [OUT] !

STACKS (A) STATE 12
STACKS (B) STATE (EMPTY)

! STACK (A) MIRRORED 2 TO STACK (B) !
RECURSIVE CALL FROM STACK (B)
+ 3 PUSHED TO STACK (A) TOP +
RECURSIVE CALL FROM STACK (A)
>> \0 triggered BY STACK (A)!
! STACK (A) MIRRORED 3 TO STACK (B) !
RECURSIVE CALL FROM STACK (B)
>> \0 triggered BY STACK (B)!
! STACK (B) POPPED 3 [OUT] !

STACKS (A) STATE 12
STACKS (B) STATE 2

! STACK (B) POPPED 2 [OUT] !

STACKS (A) STATE 1
STACKS (B) STATE (EMPTY)

! STACK (A) MIRRORED 1 TO STACK (B) !
RECURSIVE CALL FROM STACK (B)
+ 2 PUSHED TO STACK (A) TOP +
RECURSIVE CALL FROM STACK (A)
+ 3 PUSHED TO STACK (A) TOP +
RECURSIVE CALL FROM STACK (A)
>> \0 triggered BY STACK (A)!
! STACK (A) MIRRORED 3 TO STACK (B) !
RECURSIVE CALL FROM STACK (B)
>> \0 triggered BY STACK (B)!
! STACK (B) POPPED 3 [OUT] !

STACKS (A) STATE 12
STACKS (B) STATE 1

! STACK (A) MIRRORED 2 TO STACK (B) !
RECURSIVE CALL FROM STACK (B)
+ 3 PUSHED TO STACK (A) TOP +
RECURSIVE CALL FROM STACK (A)
>> \0 triggered BY STACK (A)!
! STACK (A) MIRRORED 3 TO STACK (B) !
RECURSIVE CALL FROM STACK (B)
>> \0 triggered BY STACK (B)!
! STACK (B) POPPED 3 [OUT] !

STACKS (A) STATE 12
STACKS (B) STATE 12

! STACK (B) POPPED 2 [OUT] !

STACKS (A) STATE 1
STACKS (B) STATE 1

! STACK (B) POPPED 1 [OUT] !

STACKS (A) STATE (EMPTY)
STACKS (B) STATE (EMPTY)

Answer 2

Dear Kenji,
Before any explanation by me, I strongly recommend you to take your time and visit the following link which comprehensively illustrates the whole process.
[http://cryptroix.com/2016/10/10/understanding-multiple-recursion/]

I heavily tweaked the code in order to log each step of the process to console.

#include <cstdio>
#include <string>
#include <iostream>

std::string gStack_A = ""; // STACK A
std::string gStack_B = ""; // STACK B

void abc(char *s, bool A = false, bool B = false);

void abc(char *s, bool A, bool B) {
if(A) printf("\nRECURSIVE CALL FROM STACK (A)");
else if(B) printf("\nRECURSIVE CALL FROM STACK (B)");
else printf("\nRECURSION STARTED!\n");

if (s[0] == '\0') { // poping condition
if(A)
printf("\n>> \\0 triggered BY STACK (A)!");
else
printf("\n>> \\0 triggered BY STACK (B)!");
return;
}
gStack_A += s[0];
printf("\n+ %c PUSHED TO STACK (A) TOP +", s[0]);
abc(s+1, A = true, B = false); // STACK A CALLER
printf("\n! STACK (A) MIRRORED %c TO STACK (B) !", s[0]);

gStack_B += s[0];
abc(s+1, A = false, B = true); // STACK B CALLER
printf("\n! STACK (B) POPPED %c [OUT] !", s[0]); // What you see in the output.

gStack_A.resize(gStack_A.length() - 1);
gStack_B.resize(gStack_B.length() - 1);

std::cout << "\n\nSTACKS (A) STATE " << (gStack_A != "" ? gStack_A : "(EMPTY)");
std::cout << "\nSTACKS (B) STATE " << (gStack_B != "" ? gStack_B : "(EMPTY)");
printf("\n");
} // poping point

int main() {
abc("123");
}

CODE : [https://code.sololearn.com/cy7qWXQtFoY0]

Answer 3

Are you asking for the output or for the output explanation?
The output is 3323321. If you don't understand the output I recommended you to debug step by step, you can use a C online compiler.