Big O notation for triangular numbers?

Question

What\'s the correct big O notation for an algorithm that runs in triangular time? Here\'s an example:

func(x):
  for i in 0..x
    for j in 0..i
      do_som         


        

Answer 1

The computation time increases by the factor of N*(N + 1)/2 for this code. This is essentially O(N^2).