In less-than-linear time, find the duplicate in a sorted array

Question

Today, an interviewer asked me this question. My immediate response was that we could simply do a linear search, comparing the current element with the previous element in t

Answer 1

Can be done in O(log N) with a modified binary search:

Start in the middle of the array: If array[idx] < idx the duplicate is to the left, otherwise to the right. Rinse and repeat.