[LeetCode]Find All Duplicates in an Array

Input:
[4,3,2,7,8,2,3,1]

Output:
[2,3]

public class Solution {
    public List<Integer> findDuplicates(int[] nums) {
        //require
        int size=nums.length;
        Set<Integer> set=new HashSet<Integer>();
        //invariant
        int i=0;
        while(i<size){
            if(nums[i]!=i+1){
                if(nums[nums[i]-1]==nums[i]){
                    set.add(nums[i]);
                    i++;
                }else
                    swap(i,nums[i]-1,nums);
            }else
                i++;
        }
        //ensure
        return new LinkedList<Integer>(set);
    }
    private void swap(int a,int b,int[] nums){
        int tmp=nums[a];
        nums[a]=nums[b];
        nums[b]=tmp;
    }
}