LeetCode Find the Duplicate Number

 1 public class Solution {
 2     public int findDuplicate(int[] nums) {
 3         if(nums == null || nums.length == 0){
 4             return 0;
 5         }
 6         for(int i = 0; i<nums.length; i++){
 7             int ind = Math.abs(nums[i]);
 8             if(nums[ind] < 0){
 9                 return ind;
10             }
11             nums[ind] = -nums[ind];
12         }
13         return 0;
14     }
15 }