Count triplets with sum smaller than X

Given an array arr[] of distinct integers of size N and a value sum, the task is to find the count of triplets (i, j, k), having (i<j<k) with the sum of (arr[i] + arr[j] + arr[k]) smaller than the given value sum.

• Time : O(n^2)
• Space : O(1)

c++

class Solution{
public:
    long long countTriplets(long long arr[], int n, long long sum) {
        long long res = 0;
        sort(arr, arr + n);
        for(int i = 0; i < n; i++) {
            long long l = i + 1, r = n - 1, target = sum - arr[i];
            while(l < r) {
                while(l < r and arr[l] + arr[r] >= target) r--;
                if(l < r)
                    res += (r - l);
                l++;
            }
        }
        return res;
    }
};