Code

 Header...
// INCOMPLETE

// Very simple binary tree

#define N 8 

typedef struct tree {
  struct tree *before;
  struct tree *after;
  char string[N];
}Tree;

// Add a node to the tree, return the base of the tree;
 Tree* addstring(Tree *base,const char *string){
   ... 

»  if (base) {
»    if (strcmp(string, base->string)==0) {
»      fprintf(stderr, "Repeated string %s\n", string);
»      return base;
}
    
»    if (base->before!=NULL&&strcmp(string, base->before->string)<0) {
»      addstring(base->before, string);
»      return base;
}
    else if (base->after!=NULL&&strcmp(string, base->after->string)>0) {
»      addstring(base->after, string);
»      return base;
}
  }

  // We need to put a new entry here
»  Tree *tmp;
»  tmp=xmalloc(sizeof *tmp);
»  tmp->before=tmp->after=NULL;
»  tmp->string(N-1)='\0';

»  if (base==NULL) {// First node in tree
»    strncpy(tmp->string, string, N-1);
»    return tmp;
  }

»  if (strcmp(string, base->string)>0) {
    // new word is after base
»    if (base->after==NULL) {
      // Insert new word beneath
»      base->after=tmp;
»      strncpy(tmp->string, string, N-1);
}
    else {
      // Put new text in current base node, insert tmp beneath and before it
»      strncpy(tmp->string, base->string, N-1);
»      strncpy(base->string, string, N-1);
»      tmp->before=base->before;
»      base->before=tmp;
}
  }
  else {
»    if (base->before==NULL) {
»      base->before=tmp;
»      strncpy(tmp->string, string, N-1);
}
    else {
»      strncpy(tmp->string, base->string, N-1);
»      strncpy(base->string, string, N-1);
»      tmp->after=base->after;
»      base->after=tmp;
}
  }

»  return base;
}

 int main(){
   ... 

»  Tree *t=NULL;

»  t=addstring(t, "Hello");
»  t=addstring(t, "world");
»  t=addstring(t, "Ahh");
»  t=addstring(t, "Mmmm..");

»  return 0;
}