Finding out the duplicate element in an array

Question

There is an array of size n and the elements contained in the array are between 1 and n-1 such that each element occurs once and just one element occurs more than once. We

Answer 1

Adding the elements is perfectly fine you just have to take mod(%) of the intermediate aggregate when calculating the sum of the elements and the expected sum. For the mod operation you can use something like 2n. You also have to fix the value after substraction.