Fix warnings on size_t to int conversions (#933)

Author: ShadowMitia

contents/tree_traversal/code/cpp/tree_example.cpp

@@ -11,7 +11,7 @@ using std::size_t;
 
 struct node {
   std::vector<node> children;
-  int value;
+  size_t value;
 };
 
 // Simple recursive scheme for DFS