Method twoSum

1. Two Sum/Solution.cpp:14–21

O(n) using hashmap

public:
    // O(n) using hashmap
    vector<int> twoSum(vector<int>& nums, int target) {
        unordered_map<int, int> map;
        for (int i=0; i<nums.size(); i++) {
            auto x = map.find(target - nums[i]);
            if (x != map.end()) return {x->second, i};
            map[nums[i]] = i;
        }
    }
};

int main() {