259. 3Sum Smaller

Given an array of n integers nums and a target, find the number of index triplets i, j, k with 0 <= i < j < k < n that satisfy the condition nums[i] + nums[j] + nums[k] < target.

Example:

Input: nums = [-2,0,1,3], and target = 2
Output: 2 
Explanation: Because there are two triplets which sums are less than 2:
             [-2,0,1]
             [-2,0,3]

Follow up: Could you solve it in O(n2) runtime?

int threeSumSmaller(vector<int>& nums, int target) { // time: O(n^2); space: O(1)
    sort(nums.begin(), nums.end());
    int res = 0, n = nums.size();
    for (int i = 0; i < n - 2; ++i) {
        int l = i + 1, r = n - 1;
        while (l < r) {
            int sum = nums[i] + nums[l] + nums[r];
            if (sum < target) {
                res += (r - l);
                ++l;
            } else {
                --r;
            }
        }
    }
    return res;
}