input: Write a function to find the largest number in a list.
  assert largestNumber([5, 10, 2, 9, 6]) == 10
  assert largestNumber([1, 3, 5, 7, 9]) == 9
  assert largestNumber([-1, -5, -3, -4]) == -1
output: def largestNumber(lst):
      return max(lst)