message: Update recover-a-tree-from-preorder-traversal.cpp
diff: @@ -21,7 +21,7 @@ public:
  int j = S.find_first_not_of("-", i);
  int level = j - i;
  i = j;
  - while (stack.size() != level) {
  + while (stack.size() > level) {
  stack.pop_back();
  }
  j = S.find_first_of("-", i);
  