import java.util.*; public class Solution { public int duplicate(int[] nums) { for (int i = 0; i < nums.length; i++) { while (nums[i] != i) { if (nums[i] == nums[nums[i]]) { return nums[i]; } swap(nums, i, nums[i]); } swap(nums, i, nums[i]); } return -1; } private void swap(int[] nums, int i, int...