input: Write a function to find the largest number in a list.
  assert largestNumber([4, 8, 2, 6, 9]) == 9
  assert largestNumber([10, 20, 30, 40, 50]) == 50
  assert largestNumber([-1, -4, -2, -7, -5]) == -1
output: def largestNumber(lst):
      return max(lst)