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
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