Use reserve() to pre-allocate for push_back() iteration

src/Forest.h

@@ -147,20 +147,23 @@ class Forest {
 
   std::vector<std::vector<size_t>> getNumSamplesNodes() {
     std::vector<std::vector<size_t>> result;
+    result.reserve(trees.size());
     for (auto& tree : trees) {
       result.push_back(tree->getNumSamplesNodes());
     }
     return result;
   }
   std::vector<std::vector<double>> getNodePredictions() {
     std::vector<std::vector<double>> result;
+    result.reserve(trees.size());
     for (auto& tree : trees) {
       result.push_back(tree->getNodePredictions());
     }
     return result;
   }
   std::vector<std::vector<double>> getSplitStats() {
     std::vector<std::vector<double>> result;
+    result.reserve(trees.size());
     for (auto& tree : trees) {
       result.push_back(tree->getSplitStats());
     }