Finding indices of two elements with the required sum

Given an array of integers arr and an integer sumTarget, return the indices of two elements that sum up to sumTarget.

Example 1: Input: arr = [2,7,11,15], sumTarget = 9 Output: [0,1] Explanation: arr[0] + arr[1] == 9 → return [0, 1]

Example 2: Input: arr = [3,2,4], sumTarget = 6 Output: [1,2]

Example 3: Input: arr = [3,3], sumTarget = 6 Output: [0,1]

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