1. Two Sum

Given an array of integers, returnindicesof the two numbers such that they add up to a specific target.

You may assume that each input would haveexactlyone solution, and you may not use thesameelement twice.

Example:

Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

Thoughts:
map key-value pair of component of the sum

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        unordered_map <int, int> map;
        vector <int> ans; 
        for (int i = 0; i< nums.size(); i++){
            int res = target - nums[i];
            if (map.find(res) != map.end()){
                ans.push_back(map[res]);
                ans.push_back(i);
                return ans;
            }
            map[nums[i]] = i;
        }

        return ans;
    }
};

Alternatively, I can first sort the vector _nums _and use two pointer (but the time complexity becomes sort dominant O(nlogn))