Is anyone familiar with Einstein's way of summing integers from 1 to n? I am reading the "Heard on The Street" book, which contains Q&A of the interviews for a quant role. And in answers they give the general Gauss methodology of summing integers and then proceed to explain Einstein's way. But the explanation is terrible in my opinion and I cannot wrap my head around it. Would anyone be willing to explain it? (It deals with arrays, they demonstrate the technique on series from 1 to 100)
EDIT: see my answer below for the descriptions of each method.