message: Update check-completeness-of-a-binary-tree.cpp
diff: * };
  */
  class Solution {
  +public:
  + bool isCompleteTree(TreeNode* root) {
  + bool end = false;
  + vector<TreeNode*> current{root};
  + while (current.empty() == false) {
  + vector<TreeNode*> next_level;
  + for (const auto& node : current) {
  + if (!node) {
  + end = true;
  + continue;
  + }
  + if (end) {
  + return false;
  + }
  + next_level.emplace_back(node->left);
  + next_level.emplace_back(node->right);
  + }
  + current = move(next_level);
  + }
  + return true;
  + }
  +};
  +
  +// Time: O(n)
  +// Space: O(w)
  +class Solution2 {
  public:
  bool isCompleteTree(TreeNode* root) {
  vector<pair<TreeNode*, int>> prev_level, current{{root, 1}};
  