2824. Count Pairs Whose Sum is Less than Target

class Solution {
public:
    int countPairs(vector<int>& arr, int target) {
        int n = arr.size(), count = 0, start = 0, end = n - 1;
        sort(arr.begin(), arr.end());
        
        while(start < end)
        {
            if (arr[end] + arr[start] >= target)
            {
                end--;
            }
            else
            {
                count += end - start;
                start++;
            }
        }
        return count;
    }
};
//CodeGenius