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.

For example, given nums = [-2, 0, 1, 3], and target = 2.

Return 2. 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?

Analyse: this problem only consider the index, rather than the value. Thus, we don't need to avoid duplicate values. 

Runtime: 13ms

class Solution {
public:
    int threeSumSmaller(vector<int>& nums, int target) {
        if (nums.size() < 3) return 0;
        
        sort(nums.begin(), nums.end());
        int result = 0;
        for (int i = 0; i < nums.size() - 2; i++) {
            int k = nums.size() - 1;
            for (int j = i + 1; j < k; ) {
                if (nums[i] + nums[j] + nums[k] < target) {
                    result += (k - j);
                    j++;
                }
                else k--;
            }
        }
        return result;
    }
};