message: Remove Redundancy
  tree is unused value
diff: @@ -10,7 +10,6 @@ public:
  return 0;
  }
  int ladder = 2;
  - unordered_map<string, unordered_set<string>> tree;
  unordered_set<string> left = {beginWord}, right = {endWord};
  while (!empty(left)) {
  for (const auto& word : left) {
  