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.

就是計算每個i開始能夠形成多少等差數列，如果n>3，那么就可以形成n-2種不同的組合。

剩下就是計算多少個了，我用了一個boolean數組。去判斷是否已經訪問過了。

?

Constraints:

• 3 <= nums.length <= 200

• 0 <= nums[i] <= 200

• 1 <= diff <= 50

• nums?is?strictly?increasing.

Runtime:?3 ms, faster than?42.99%?of?Java?online submissions for?Number of Arithmetic Triplets.

Memory Usage:?40.6 MB, less than?52.22%?of?Java?online submissions for?Number of Arithmetic Triplets.