message: Update validate-binary-tree-nodes.cpp
diff: @@ -14,13 +14,13 @@ public:
  for (const auto& i : rightChild) {
  roots.erase(i);
  }
  - if (roots.size() != 1) {
  + if (size(roots) != 1) {
  return false;
  }
  const auto& root = *cbegin(roots);
  vector<int> stk({root});
  unordered_set<int> lookup({root});
  - while (!stk.empty()) {
  + while (!empty(stk)) {
  const auto node = stk.back(); stk.pop_back();
  for (const auto& i : {leftChild[node], rightChild[node]}) {
  if (i < 0) {
  @@ -33,6 +33,6 @@ public:
  stk.emplace_back(i);
  }
  }
  - return true;
  + return size(lookup) == n;
  }
  };
  