The above answer is a typical answer or brute force technique, which anyone could answer. What they are looking for is probably how good you are at math and algorithms. My answer would be:

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.

my answer was to sort the array and use a boolean variable with a while loop that will exit when the number is not equal to the previous number plus 1