leetcode-1.twosum

class  {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        vector<int> res;
        for (int i=0;i<nums.size();i++){
            for (int j=i+1;j<nums.size();j++){
                if (nums[i]+nums[j]==target){
                    res.push_back(i);
                    res.push_back(j);
                    return res;
                }
            }
        }
    }
};

class  {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        using vType = remove_reference_t<decltype(nums)>::size_type;
        /*remove_referce_t: ????*/
        unordered_map<int,vType> m ;
        /*unordered_map uses more memory but is efficient*/
        for (int i = 0; i < nums.size(); ++i) {
        /*use find target-num[i] to find solution in O(n)*/
            const auto it = m.find(target - nums[i]);
            if (it != m.end())
                    return vector<int> {it->second,i};
            m.emplace(nums[i],i);
        }
        throw std::runtime_error("no solutions:") ;
    }
};