/*
 * This code is from "Algorithms in C, Third Edition,"
 * by Robert Sedgewick, Addison Wesley Longman, 1998.
 */
link STjoinR(link a, link b)
  { 
    if (a == z) return b;
    b = STinsert(b, a->rec); 
    b->l = STjoin(a->l, b->l); 
    b->r = STjoin(a->r, b->r); 
    fixN(b); free(a);
    return b;
  }
link STjoin(link a, link b)
  { 
    if (rand()/(RAND_MAX/(a->N+b->N)+1) < a->N)
         STjoinR(a, b);
    else STjoinR(b, a);
  }