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
Output: 2
Explanation: Because there are two triplets which sums are less than 2:

Follow up: Could you solve it in \(O(n^2)\) runtime?


Key observation: the answer does not change if numbers are swapped. Thus we can sort the numbers first and use a sliding window method.

We count the number of such combinations for each \(i\), so that we can reduce the problem to 2Sum smaller. For 2Sum Smaller, we initialize two indices, left and right, pointing to the first and last element respectively. If nums[left] + nums[right] >= target - nums[i], then there are no combinations with i and k = right (because j = left is the smallest), and we need to reduce right. Else, there are right - left pair of (j, k) when j = left, and we can add them to the number of combinations and increment left.

class Solution(object):
    def threeSumSmaller(self, nums, target):
        :type nums: List[int]
        :type target: int
        :rtype: int
        if len(nums) < 3:
            return 0

        sums = 0
        for i in range(len(nums) - 2):
            sums += self.__twoSumSmaller(nums[i+1:], target - nums[i])

        return sums

    def __twoSumSmaller(self, nums, target):
        sums = 0
        left, right = 0, len(nums) - 1
        while left < right:
            if nums[left] + nums[right] < target:
                sums += right - left
                left += 1
                right -= 1

        return sums