Pasted

public class TriangularMatrix<T>
{
    private readonly T[] arr;
    private readonly int dim;
 
    public TriangularMatrix(int n)
    {
        dim = n;
        arr = new T[n * (n + 1) / 2];
    }
 
    // i = row, j = column
    public T this[int row, int col]
    {
        get { return arr[row * (row + 1) / 2 + col]; }
        set { arr[row * (row + 1) / 2 + col] = value; }
    }
 
    public T this[int i]
    {
        get { return arr[i]; }
        set { arr[i] = value; }
    }
 
    public int Rows { get { return dim; } }
    public int Columns { get { return dim; } }
    public int Length { get { return arr.Length; } }
 
    public void GetMatrixCoordinates(int i, out int row, out int col)
    {
        var root = TriangularRoot(i);
        row = (int)Math.Floor(root);
        col = i - row * (row + 1) / 2;
    }
    
    private static double TriangularRoot(double x) {
        return (Math.Sqrt(8 * x + 1) - 1) / 2;
    }
}