Creating a linked list from a binary tree (preorder tranversal)

Question

So I'm trying to create a linked list from a binary tree using preorder tranversal. I'm having so many problems at doing it, I saw some of the "solutions" but I didn't like it! I am trying something simple.

This is the code that I got until now:

typedef struct nodo {
  int value;
  struct nodo *left, *right;
} *ABin;

typedef struct lligada { 
  int value;
  struct lligada *next;
} *LInt;

void preorder (ABin a, LInt * l) {

  LInt r=*l,tmp;
  tmp=r;

  if (!a) {
    *l=NULL;
}
  else {
    r=malloc(sizeof(struct lligada));
    r->value=a->value;
    r=r->next;
    *l=tmp;
    preorder (a->left,l);
    preorder (a->right,l);
  }
}

I'm getting always an empty list!

Answer 1

if (!a) { *l=NULL; }

This will allways be the last thing done in your function, with the null in *l being passed all the way up.

The rest also has problems:

r=r->next;

But you never set r->next to anything. You'll have to do that first.

Further, when you first call preorder() what, if anything, is *l pointing to? You'd probably be better off, instead of passing in a LInt*, having the function return a Lint* (joining the lists after the internal calls to preorder())