46. Permutations

class Solution {
    public List<List<Integer>> permute(int[] nums) {
        List<List<Integer>> res = new ArrayList<>();
        if (nums.length < 1) return res;
        helper(res, new ArrayList<>(), nums, new boolean[nums.length]);
        return res;        
    }
    
    private void helper(List<List<Integer>> res, 
        List<Integer> cur, int[] nums, boolean[] visited) {
        if (cur.size() == nums.length) {
            res.add(new ArrayList(cur));
            return;
        }
        
        for (int i = 0; i < nums.length; i++) {
            if (visited[i]) continue;
            cur.add(nums[i]);
            visited[i] = true;
            helper(res, cur, nums, visited);
            cur.remove(cur.size() - 1);
            visited[i] = false;
        }
    }
}