LeetCode/cs/count-good-triplets.cs

public class Solution {
    private IEnumerable<(int, int, int)> Triplets(int n) =>
        Enumerable.Range(0, n).SelectMany(
            i => Enumerable.Range(i + 1, n - i - 1).SelectMany(
                j => Enumerable.Range(j + 1, n - j - 1).Select(
                    k => (i, j, k)
                )
            )
        );

    public int CountGoodTriplets(int[] arr, int a, int b, int c) =>
        Triplets(arr.Length)
        .Select(idxs => {
            var (i, j, k) = idxs;
            return (arr[i], arr[j], arr[k]);
        })
        .Where(nums => {
            var (x, y, z) = nums;
            return Math.Abs(x - y) <= a && Math.Abs(y - z) <= b && Math.Abs(x - z) <= c;
        })
        .Count();
}