Software Job - Interview Question

Today, I was searching for some technical articles and stumbled upon this interview question from Amazon, for which I could immediately think about some good solution. Here's the question:

We have numbers from 1 to 52 that are put into a 51 number array, what's the best way to find out which number is missing?

I read an answer from some guy, who said to sort the numbers first and then loop through and find the number. I thought that was inefficient way of doing that. My answer is as below:

1) - Calculate the sum of all numbers stored in the array of size 51. 
2) - Subtract the sum from (52 * 53)/2   -- Formula : n (n+1)/2.

The result of subtraction is the answer for this question.

Any other solutions that you know of?!?!?!