*Index pair of which the the sum of their value is given.

#include <iostream>
#include <vector>
#include <unordered_map>

using namespace std;

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        unordered_map<int, int> seenValueIndex; // whenever we observe a value, we'll keep its
        // value as key and index as value

        vector<int> pair; // resulted indexes

        for (int i = 0; i < nums.size(); i++)
        {
            int compliment = target - nums[i]; // sum - value at index
            if (seenValueIndex.find(compliment) != seenValueIndex.end())
            { // if found
                pair.push_back(seenValueIndex[compliment]);
                pair.push_back(i);
                break;
            }
            seenValueIndex[nums[i]] = i; // key = value, value = index
        }
        return pair;
    }
};

int main(void)
{
    cout << "How many elements? : ";
    int size; cin >> size;

    vector<int> list;
    for (int i = 0; i < size; i++)
    {
        cout << "\nElement NO. " << (i + 1) << " : ";
        int element;
        cin >> element;
        list.push_back(element);
    }

    cout << "\nTarget sum : ";
    int target;
    cin >> target;

    Solution s;
    vector<int> result = s.twoSum(list, target);

    cout << endl << "Resultant pair (if exists): [";

    int count = 0;
    for (int i : result)
    {
        ++count;
        if (count % 2 != 0)
        {
            cout << i + 1 << ", ";
        }
        else
        {
            cout << i + 1;
        }
    }

    if (count == 0)
        cout << "-";

    cout << "]\n\n";
    return 0;
}