code: @Override
      public float getLeafValue(FVec feat, int root_id) {
          Node n = nodes[root_id];
          while (!n._isLeaf) {
              n = nodes[n.next(feat)];
          }
  
          return n.leaf_value;
      }
docs: Retrieves nodes from root to leaf and returns leaf value.
  
  @param feat    feature vector
  @param root_id starting root index
  @return leaf value