LeetCode 46. Permutations

public class Solution {    public List<List<Integer>> permute(int[] nums) {        List<List<Integer>> list = new ArrayList<List<Integer>>();        int len = nums.length;        if (len == 1) {        List<Integer> l = new ArrayList<Integer>();        l.add(nums[0]);        list.add(l);        return list;        }        Set<Integer> set = new HashSet<Integer>();        for (int i = 0; i < len; i++) set.add(nums[i]);        for (int i = 0; i < len; i++) {        int num = nums[i];        set.remove(num);        int[] subNums = new int[len - 1];        int j = 0;        for (int n : set) subNums[j++] = n;        List<List<Integer>> subList = permute(subNums);        for (List<Integer> l : subList) l.add(num);        list.addAll(subList);        set.add(num);        }        return list;    }}