2367. Number of Arithmetic Triplets

Problem

You are given a 0-indexed, strictly increasing integer array nums and a positive integer diff. A triplet (i, j, k) is an arithmetic triplet if the following conditions are met:

• i < j < k,

• nums[j] - nums[i] == diff, and

• nums[k] - nums[j] == diff.

Return **the number of unique **arithmetic triplets.

  Example 1:

Input: nums = [0,1,4,6,7,10], diff = 3
Output: 2
Explanation:
(1, 2, 4) is an arithmetic triplet because both 7 - 4 == 3 and 4 - 1 == 3.
(2, 4, 5) is an arithmetic triplet because both 10 - 7 == 3 and 7 - 4 == 3. 

Example 2:

Input: nums = [4,5,6,7,8,9], diff = 2
Output: 2
Explanation:
(0, 2, 4) is an arithmetic triplet because both 8 - 6 == 2 and 6 - 4 == 2.
(1, 3, 5) is an arithmetic triplet because both 9 - 7 == 2 and 7 - 5 == 2.

  Constraints:

• 3 <= nums.length <= 200

• 0 <= nums[i] <= 200

• 1 <= diff <= 50

• nums is strictly increasing.

Solution (Java)

class Solution {
    public int arithmeticTriplets(int[] nums, int diff) {
        Set<Integer> set = new HashSet<>();
        for (int x : nums) {
            set.add(x);
        }
        int ans = 0;
        for (int x : nums) {
            if (set.contains(x - diff) && set.contains(x + diff)) {
                ans++;
            }
        }
        return ans;
    }
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).