Revert member to unordered_set

cpp/src/parquet/arrow/reader.cc

@@ -159,7 +159,7 @@ class FileReaderImpl : public FileReader {
     ctx->pool = pool_;
     ctx->iterator_factory = SomeRowGroupsFactory(row_groups);
     ctx->filter_leaves = true;
-    ctx->included_leaves = indices;
+    ctx->included_leaves.insert(indices.begin(), indices.end());
     return manifest_.schema_fields[i].GetReader(ctx, out);
   }
 

cpp/src/parquet/arrow/reader_internal.h

@@ -115,12 +115,11 @@ struct ReaderContext {
   ::arrow::MemoryPool* pool;
   FileColumnIteratorFactory iterator_factory;
   bool filter_leaves;
-  std::vector<int> included_leaves;
+  std::unordered_set<int> included_leaves;
 
   bool IncludesLeaf(int leaf_index) const {
     return (!this->filter_leaves ||
-            std::find(included_leaves.begin(), included_leaves.end(), leaf_index) !=
-                included_leaves.end());
+            (included_leaves.find(leaf_index) != included_leaves.end()));
   }
 };