message: Update reverse-pairs.cpp
diff: class Solution {
  public:
  int reversePairs(vector<int>& nums) {
  - return sortAndCount(nums.begin(), nums.end());
  + return countAndMergeSort(nums.begin(), nums.end());
  }
  private:
  - int sortAndCount(vector<int>::iterator begin, vector<int>::iterator end) {
  + int countAndMergeSort(vector<int>::iterator begin, vector<int>::iterator end) {
  if (end - begin <= 1) {
  return 0;
  }
  auto mid = begin + (end - begin) / 2;
  - int count = sortAndCount(begin, mid) + sortAndCount(mid, end);
  + int count = countAndMergeSort(begin, mid) + countAndMergeSort(mid, end);
  for (auto i = begin, j = mid; i != mid; ++i) {
  while (j != end && *i > 2L * *j) {
  ++j;
  