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.
Input: nums = [-2,0,1,3], and target = 2
Explanation: Because there are two triplets which sums are less than 2:
跟 3Sum 类似，但是这题只用求count个数。所以当second 和 third满足条件时，
count += third - second就可以了。