Find Duplicate in Array

Given a read only array of n + 1 integers between 1 and n, find one number that repeats in linear time using less than O(n) space and traversing the stream sequentially O(1) times.

Sample Input:

[3 4 1 4 1]

Sample Output:


If there are multiple possible answers ( like in the sample case above ), output any one.

If there is no duplicate, output -1

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