class Solution {
public:
    vector<int> numberOfPairs(vector<int>& nums) {
        set<int> temp;
        int len = nums.size();
        for (int i = 0; i < len; i++) {
            int t = nums[i];
            auto tt = temp.find(t);
            if (tt == temp.end()) temp.insert(t);
            else temp.erase(tt);
        }
        vector<int> ret(2);
        ret[1] = temp.size();
        ret[0] = (len - ret[1]) / 2;
        return ret;
    }
};