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// use sorting and unordered_set
vector<int> find_duplicate_numbers(vector<int> &nums) {
    vector<int> res;
    if(nums.empty()) return res;
    unordered_set<int> se;
    sort(nums.begin(), nums.end());
    for(int i=1; i<nums.size()-1; ++i) {
        if(nums[i] == nums[i-1] || nums[i] == nums[i+1])
            se.insert(nums[i]);
    }
    for(auto i : se) res.push_back(i);
    return res;
}

// use hash, better. 
vector<int> find_duplicate_numbers(vector<int> &nums) {
    vector<int> res;
    if(nums.empty()) return res;
    unordered_map<int, int> mp;
    for(auto i : nums) mp[i]++;
    for(auto p : mp) {
        if(p.second > 1)
            res.push_back(p.first);
    }
    return res;
}