log.debug("Building {} trees", nbTrees);
for (int treeId = 0; treeId < nbTrees; treeId++) {
log.debug("Building tree number : {}", treeId);
Node tree = bagging.build(rng);
key.set(partition, firstTreeId + treeId);
if (isOutput()) {
MapredOutput emOut = new MapredOutput(tree);