input: Write a function to find the missing number in an array containing numbers from 1 to n with one number missing.
  assert find_misssing_number([9, 5, 4, 6, 8, 7, 1, 2]) == 3
  assert find_misssing_number([1, 2, 4, 5, 6]) == 3
  assert find_misssing_number([3, 7, 1, 2, 8, 4, 5]) == 6
output: def find_misssing_number(arr):
      n = len(arr) + 1
      total_sum = n * (n + 1) // 2
      arr_sum = sum(arr)
      return total_sum - arr_sum