message: Update merge-similar-items.cpp
diff: // Time: O((m + n) * log(m + n))
  // Space: O(m + n)
  -// freq table
  +// freq table, sort
  class Solution {
  public:
  vector<vector<int>> mergeSimilarItems(vector<vector<int>>& items1, vector<vector<int>>& items2) {
  