README.md

2367. Number of Arithmetic Triplets

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.

Solutions (Rust)

1. Solution

use std::collections::HashSet;

impl Solution {
    pub fn arithmetic_triplets(nums: Vec<i32>, diff: i32) -> i32 {
        let mut nums_set = HashSet::from([nums[0], nums[1]]);
        let mut ret = 0;

        for &num in &nums[2..] {
            nums_set.insert(num);

            if nums_set.contains(&(num - diff)) && nums_set.contains(&(num - 2 * diff)) {
                ret += 1;
            }
        }

        ret
    }
}