1.从双倍数组中还原原数组（力扣，vector）

java式c++解法。

class Solution {
public:
    vector<int> findOriginalArray(vector<int>& changed) {
        int n = changed.size();
        if(n % 2 == 1) return {};
        map<int, int> mp;
        for(int c : changed) mp[c]++;
        sort(changed.begin(), changed.end());
        vector<int> ans;
        int i = 0;
        while(i < n){
            int x = changed[i];
            if(mp[x] > 0){
                mp[x]--;
                if(mp[x * 2] <= 0) return {};
                mp[x * 2]--;
                ans.push_back(x);
            }
            i++;
        }
        return ans;
    }
};

2.