partitions.add(partition);
int nbTrees = Step1Mapper.nbTrees(NUM_MAPS, NUM_TREES, partition);
for (int treeId = 0; treeId < nbTrees; treeId++) {
Node tree = new Leaf(rng.nextInt(100));
keys[index] = new TreeID(partition, treeId);
values[index] = new MapredOutput(tree, nextIntArray(rng, NUM_INSTANCES));
index++;