code: Map<Integer, Integer> initParentMap() {
          Map<Integer, Integer> parentMap = new LinkedHashMap<>(numWorkers);
          for(int r = 0; r < numWorkers; r++) {
              parentMap.put(r, ((r + 1) / 2 - 1) );
          }
          return parentMap;
      }
docs: Generates a mapping node -> parent (parent of root is -1)