Sum of Triangular Numbers

Sum of Triangular NumbersCodewars

Approach

end number of each line of number pyramid is *previous-last-number + line number

which means the 5th end number will *previous-last-number(4th) + 4

= (4th last number is 10) + 5 = 15

Solution

JavaScript

const sumTriangularNumbers = (n, count = 1, previousEndNum = 0, sum = 0) => {
  return count <= n ? sumTriangularNumbers(
    n,
    count + 1,
    previousEndNum + count,
    sum + previousEndNum + count
  ) : sum;
};

time complexity is O(n)