923 - 3Sum With Multiplicity\.

Medium

Given an integer array arr, and an integer target, return the number of tuples i, j, k such that i < j < k and arr[i] + arr[j] + arr[k] == target.

As the answer can be very large, return it modulo <code>10⁹ + 7</code>.

Example 1:

Input: arr = 1,1,2,2,3,3,4,4,5,5, target = 8

Output: 20

Explanation:

Enumerating by the values (arr[i], arr[j], arr[k]):
(1, 2, 5) occurs 8 times;
(1, 3, 4) occurs 8 times;
(2, 2, 4) occurs 2 times;
(2, 3, 3) occurs 2 times.

Example 2:

Input: arr = 1,1,2,2,2,2, target = 5

Output: 12

Explanation:

arri = 1, arrj = arrk = 2 occurs 12 times:

We choose one 1 from 1,1 in 2 ways,

and two 2s from 2,2,2,2 in 6 ways.

Constraints:

• 3 <= arr.length <= 3000

• 0 <= arr[i] <= 100

• 0 <= target <= 300