Three Sum With Multiplicity LeetCode (Array 3 Sum with Multiplicity)

Given an integer array A, and an integer target, return the number of tuples i, j, k  such that i < j < k and A[i] + A[j] + A[k] == target. As the answer can be very large, return it modulo 10^9 + 7.

Example 1:

1. Input: A = [1,1,2,2,3,3,4,4,5,5], target = 8
2. Output: 20
3. Explanation: 
1. Enumerating by the values (A[i], A[j], A[k]):
2. (1, 2, 5) occurs 8 times;
3. (1, 3, 4) occurs 8 times;
4. (2, 2, 4) occurs 2 times;
5. (2, 3, 3) occurs 2 times.

Example 2:

1. Input: A = [1,1,2,2,2,2], target = 5
2. Output: 12
3. Explanation: 
1. A[i] = 1, A[j] = A[k] = 2 occurs 12 times:
2. We choose one 1 from [1,1] in 2 ways,
3. and two 2s from [2,2,2,2] in 6 ways.

NOTE:

• 3 <= A.length <= 3000
• 0 <= A[i] <= 100
• 0 <= target <= 300

This is an interesting problem that is also discussed in LeetCode and GeeksForGeeks A collection of hundreds of interview questions and solutions are available in our blog at Interview Question

Solution