Find the Missing Number 1 to n

You are given a list of n-1 integers and these integers are in the range of 1 to n. There are no duplicates in list. One of the integers is missing in the list. Write an efficient code to find the missing integer.

Example:
I/P    [1, 2, 4, ,6, 3, 7, 8]
O/P    5

METHOD 1(Use sum formula)
Algorithm:

1. Get the sum of numbers 
       total = n*(n+1)/2
2  Subtract all the numbers from sum and
   you will get the missing number.

Example::

total = n*(n+1)/2 = 8*9/2 = 36

sum = 31

Missing number = total - sum =  36-31 = 5