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# Find Duplicate in Array

Problem Description

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.

If there are multiple possible answers ( like in the sample case ), output any one, if there is no duplicate, output -1

Problem Constraints
1 <= |A| <= 105
1 <= Ai <= |A|

Input Format
The first argument is an integer array A.

Output Format
Return the integer that repeats in array A

Example Input
Input 1:
`A = [3, 4, 1, 4, 2]`
Input 2:
`A = [1, 2, 3]`
Input 3:
`A = [3, 4, 1, 4, 1]`

Example Output
Output 1:
`4`
Output 2:
`-1`
Output 3:
`1`

Example Explanation
Explanation 1:
`4 repeats itself in the array [3, 4, 1, 4, 2]`
Explanation 2:
`No number repeats itself in the array [1, 2, 3]`
Explanation 3:
`1 and 4 repeats itself in the array [3, 4, 1, 4, 1], we can return 1 or 4`

NOTE: You only need to implement the given function. Do not read input, instead use the arguments to the function. Do not print the output, instead return values as specified. Still have a question? Checkout Sample Codes for more details.
Hints
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## Discussion

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