public class Solution{
    public List<List<Integer>> permutations(int[] arr){
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        //corner
        if ( arr == null || arr.length == 0 ){
            return res;
        }

        //core
        List<Integer> curList = new ArrayList<>();

        helper(res, curList, arr);

        return res;
    }


    public void helper(List<List<Integer>> res, List<Integer> curList, int[] arr){
        //base
        if ( curList.size() == arr.length ){
            res.add(new ArrayList<Integer>(curList));           
            return ;
        }

        //current
        for ( int i = 0; i < arr.length; i++ ){
            if ( curList.contains(arr[i]) ){
                continue;
            }

            curList.add(arr[i]);

            //next
            helper(res, curList, arr);

            curList.remove(curList.size() - 1);                   
        }
    }
}

排列组合算法