You are given a 0-indexed, strictly increasing integer array numsand 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

(资料图片仅供参考)

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.

就是计算每个i开始能够形成多少等差数列，如果n>3，那么就可以形成n-2种不同的组合。

剩下就是计算多少个了，我用了一个boolean数组。去判断是否已经访问过了。

Constraints:

3 <= nums.length <= 200

0 <= nums[i] <= 200

1 <= diff <= 50

numsis 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.