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:

1

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

If there is no duplicate, output -1

public class Solution {
    public int repeatedNumber(final int[] A) {
        HashMap<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < A.length; i++) {
            if (!map.containsKey(A[i]))
                map.put(A[i], 1);
            else
                return A[i];
        }
        return -1;
    }
}