Problem Statement

Given an array arr of unsorted numbers and a target sum, count all triplets in it such that arr[i] + arr[j] + arr[k] < target where i, j, and k are three different indices. Write a function to return the count of such triplets.

Example 1:

Input: [-1, 0, 2, 3], target=3 
Output: 2
Explanation: There are two triplets whose sum is less than the target: [-1, 0, 3], [-1, 0, 2]

Example 2:

Input: [-1, 4, 2, 1, 3], target=5 
Output: 4
Explanation: There are four triplets whose sum is less than the target: 
[-1, 1, 4], [-1, 1, 3], [-1, 1, 2], [-1, 2, 3]

Constraints:

• n == att.length
• 0 <= n <= 3500
• -100 <= arr[i] <= 100
• -100 <= target <= 100

Try it yourself

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