// Time:  O(n)
// Space: O(n)

class TwoSum {
public:

    // Add the number to an internal data structure.
    void add(int number) {
        ++lookup_[number];
    }

    // Find if there exists any pair of numbers which sum is equal to the value.
    bool find(int value) {
        for (const auto& kvp : lookup_) {
            const auto num = value - kvp.first;
            if (lookup_.count(num) && (num != kvp.first || kvp.second > 1)) {
                return true;
            }
        }
        return false;
    }

private:
    unordered_map<int, int> lookup_;
};


// Your TwoSum object will be instantiated and called as such:
// TwoSum twoSum;
// twoSum.add(number);
// twoSum.find(value);