class Solution {

    public int findDuplicate(int[] nums) {

        if(nums == null || nums.length<=1)

            return -1;

        int fast = 0, slow = 0;

        while (true){

            fast = nums[nums[fast]];

            slow = nums[slow];

            if(fast == slow){

                fast = 0;

                while(nums[slow] != nums[fast]){

                    fast = nums[fast];

                    slow = nums[slow];

                }

                return nums[slow];

            }

        }

    }

}