Find Duplicate Number in Array in Java

Given an array of sizen + 1 with elements from 1 to n. One element is duplicated multiple times. Find that element in O(1) space. An array cannot be changed.
publicclassDuplicateNumberDetection {publicintfindDuplicate(int[]nums) {if (nums.length==0|| nums.length==1) {return-1;        }int slow= nums[0];int fast= nums[nums[0]];while (slow!= fast) {            slow= nums[slow];            fast= nums[nums[fast]];        }        fast=0;while (slow!= fast) {            slow= nums[slow];            fast= nums[fast];        }return fast;    }publicstaticvoidmain(Stringargs[]) {int[] input= {2,1,3,1,5,4};DuplicateNumberDetection dd=newDuplicateNumberDetection();System.out.println(dd.findDuplicate(input));    }}
Output:
1

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