Input: nums = [2,7,11,15], target = 9
Output: [0,1]
Explanation: Because nums[0] + nums[1] == 9, we return [0, 1].

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        
    }
};

      10
    / | \
   5  3  6


 i  j
[2, 7, 11, 15, ], target = 18
output: [1, 2]

[2, 2], target = 4

forloop i 
    forloop j
     var sum = num[i] + num[j];
     if (sum = ta)

n + (n - 1) + (n -2) ... + 1

(n+1)*n/2 = n^2

forloop i
    map[target - num[i]] = i;
    
forloop j
    if(!map[nums[j]].empty) {
        int pairIndex = map[nums[j]];
        return [j, pairIndex];
    }
    
    O(n) O(n)