## DEV Community

Abhishek Chaudhary

Posted on

Given a 0-indexed integer array `nums`, return the number of distinct quadruplets `(a, b, c, d)` such that:

• `nums[a] + nums[b] + nums[c] == nums[d]`, and
• `a < b < c < d`

Example 1:

Input: nums = [1,2,3,6]
Output: 1
Explanation: The only quadruplet that satisfies the requirement is (0, 1, 2, 3) because 1 + 2 + 3 == 6.

Example 2:

Input: nums = [3,3,6,4,5]
Output: 0
Explanation: There are no such quadruplets in [3,3,6,4,5].

Example 3:

Input: nums = [1,1,1,3,5]
Output: 4
Explanation: The 4 quadruplets that satisfy the requirement are:

• (0, 1, 2, 3): 1 + 1 + 1 == 3
• (0, 1, 3, 4): 1 + 1 + 3 == 5
• (0, 2, 3, 4): 1 + 1 + 3 == 5
• (1, 2, 3, 4): 1 + 1 + 3 == 5

Constraints:

• `4 <= nums.length <= 50`
• `1 <= nums[i] <= 100`

SOLUTION:

``````class Solution:
def countQuadruplets(self, nums: List[int]) -> int:
n = len(nums)
ctr = 0
exists = {}
for i, num in enumerate(nums):
exists[num] = exists.get(num, []) + [i]
for i in range(n):
for j in range(i + 1, n):
for k in range(j + 1, n):
val = nums[i] + nums[j] + nums[k]
if val in exists:
for p in exists[val]:
if p > k:
ctr += 1
return ctr
``````