2824. Count Pairs Whose Sum is Less than Target 

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class Solution {
    public int countPairs(List<Integer> nums, int target) {
        Collections.sort(nums);

        int count = 0;
        int i = 0, j = nums.size() - 1;

        while (i < nums.size()) {
            while (i < j && nums.get(i) + nums.get(j) >= target) {
                j--;
            }

            if (i == j) {
                break;
            } else {
                count += j - i;
            } 

            i++;
        }

        return count;
    }
}

References

2824. Count Pairs Whose Sum is Less than Target