class Solution {
public:
    int countPairs(vector<int>& nums, int target) {
        int res = 0;
        int n = nums.size();
        sort(nums.begin(), nums.end());
        for (int i = 0, j = n - 1; i < j; ) {
            if (nums[i] + nums[j] < target) {
                res += j - i;
                i ++ ;
            } else j -- ;
        }
        return res;
    }
};