std::vector<int>Sort(std::vector<int>&arr) { assert(!arr.empty()); std::unordered_map<int, int>freqMap; for (auto it : arr) { auto iter = freqMap.find(it); if (iter != freqMap.end()) { freqMap[it]++; } else freqMap[it] = 1;   } std::sort(arr.begin(), arr.end(), [&freqMap](int lhs, int rhs){ return freqMap.at(lhs) > freqMap.at(rhs); }); return arr; }