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?