287. Find the Duplicate Number

 1 class Solution {
 2     public int findDuplicate(int[] nums) {
 3         if(nums.length <= 1) return -1;
 4         int slow = nums[0];
 5         int fast = nums[nums[0]];
 6         while(slow != fast){
 7             slow = nums[slow];
 8             fast = nums[nums[fast]];
 9         }
10         fast = 0;
11         while(fast != slow){
12             fast = nums[fast];
13             slow = nums[slow];
14         }
15         return fast;
16         
17     }
18 }