python3.6

Write a function trace that takes a square 2D list as an argument and returns the sum of the diagonal elements. trace(l) l:list(list(int)), return: int. Assume len(l) = len(l[1]). >>> l = [[1,2,3],[4,5,6],[7,8,9]] // 1 + 5 + 9 = 15

>>> trace(l)

15

>>> l = [[1]]

>>> trace(l)

1

>>> l =[[1,2],[3,4]] // // 1 + 4 = 5

>>> trace(l)

5

>>> l = [[1,2,3,4],[5,6,7,8],[9,10,11,12], [13,14,15,16]] // 1 + 6 + 11 + 15 = 34

>>> trace(l)

34