2221. Find Triangular Sum of an Array

// Approach: Iteratively reduce array by pairwise modular sums until one element remains.
// Time: O(n²) Space: O(1)

public class Solution
{
    public int TriangularSum(int[] nums)
    {
        for (int currentLength = nums.Length - 1; currentLength > 0; currentLength--)
        {
            for (int index = 0; index < currentLength; index++)
                nums[index] = (nums[index] + nums[index + 1]) % 10;
        }
        return nums[0];
    }
}