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So as homework, I need to implement the reverseRec function recursively. I can't use any loops, but must use the putBack function in it and the function needs to return something, which makes everything unnecessarily complicated.

#include "stdio.h"
#include "stdlib.h"

int length(char *s) {
  int n = 0;
  while(*s != '\0') {
  return n;

void copy(char* s, int n, char* t) {
  int i = 0;
  while(i < n) {
    t[i] = s[i];

char* putBack(char* s, char c) {
  const int n =  length(s);
  char* r = malloc(sizeof(char) * (n+2));
  copy(s, n, r);
  r[n] = c;
  r[n+1] = '\0';
  return r;

char* reverseRec(char *s){     

        s = putBack(s, *(s-1));           //???
    return s;

int main() {
printf("%s", reverseRec("ABC"));

What I have tried:

So after the recursion reaches the end of the string, it returns. If e.g. the string is "ABC", then it goes ABC-->BC-->C-->/0-->C-->BC-->ABC. My idea was, that I somehow save the beginning of the string (*s) of every returning recusion, and add them together with the putBack funcion to CBA. But I think this can't work, since it's a recursion, which means I can't save anything from any recursion depth, because it would be overritten, right? I feel like I'm missing something about how recursions and buffers work in C, but idk.
Updated 21-May-20 6:44am

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When recursion is explicit your task than you will need to use the divide and conquer algorithm.

A possible solution may be that you call your swap function with the first and last char* of your string, check that their distance is greater than 1 (uneven case) than swap and call recursive the function with the next swap pair.

void swap(char *first, char* last)
  int diff = last - first;

  if( diff > 1 ) {
    swap(first + 1, last - 1);
  char c = *first;
  *last = *last;
  *first= c;
I hope you understand the principle.
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example input string "12345" must become "54321"
your prof gave you a hint about. The recursiveness breaks the input string into smaller parts and then builds then 1 by 1 together. This means the amount of "recursive steps" are five for the input string "12345"

Hint 1: the inputs for reverseRec( ) are
input 1: "12345"
input 2: "2345"
input 3: "345"
input 4: "45"
input 5: "5"

Hint 2: the outputs from each reverseRec( ) are
output 1: "54321"
output 2: "5432"
output 3: "543"
output 4: "54"
output 5: "5"

Hint 3: since reverseRec( ) calls itself, it returns itself a value. This value can be used.
reverseRec 1( string ) gets "5434"
reverseRec 2( string ) gets "543" ( the called reverseRec3 returns "543" )
reverseRec 3( string ) gets "54" ( the called reverseRec4 returns "54" )
reverseRec 4( string ) gets "5" ( the called reverseRec5 returns "5" )
reverseRec 5( string ) gets nothing ( no next reverseRec was called to get a value from)

Hint 4: now you got a set of information and a missing objective
given information (inputs) from Hint 1:
given information (outputs) from Hint 2:
returned value from Hint 3:

reverseRec 1( string ) gets "5434", should return "54321", given input string "12345"
reverseRec 2( string ) gets "543", should return "5432", given input string "2345"
reverseRec 3( string ) gets "54", should return "543", given input string "345"
reverseRec 4( string ) gets "5", should return "54", given input string "45"
reverseRec 5( string ) gets nothing, returns 5, given input "5"

Hint 5: the missing objective
you must create the [should return "output"] for step 4,3,2,1
you got the return value, the input string, and putBack

The information is 80% of the work ;) now sit down and test some ideas with putback,
or use the debugger to go step by step to see what is happening when.

reverseRec( ) {
// recursive magic
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