int[] arr = {1, 2, 3}
sub array 必须是连续的元素，比如{1}, {1,2}, {1,2,3}，不能有{1,3}
sub sequence or subset 中元素可以不连续， 比如{1,3}
permutation意思为排列，元素出现顺序不同即为不同permutation
combination意思为组合，元素出现顺序不同也可以为同一个组合

import java.util.ArrayList;
import java.util.List;

public class SubarrayAndSubSequenceAndPermutation {

    public static void main(String[] args) {
        int[] arr = {1,2,3};

        //generate all the subarrays
        List<List<Integer>> li = new ArrayList<>();
        for (int i = 0; i < arr.length; i++) {
            List<Integer> temp = new ArrayList<>();
            for (int j = i; j < arr.length; j++) {
                 temp.add(arr[j]);
                 li.add(new ArrayList<>(temp));
            }

        }
        System.out.println(li);

        //subsequence Or Combination!!!!!!!!!
        List<List<Integer>> subsequence = new ArrayList<>();
        generateSubSequence(subsequence, new ArrayList<>(), arr, 0);
        System.out.println(subsequence);


        List<List<Integer>> permutationList = new ArrayList<>();
        generatePermutation(permutationList, new ArrayList<>(), arr);
        System.out.println(permutationList);
    }

    //store the index of element in orginal array in case there are redundent elements in the array
    private static void generatePermutation(List<List<Integer>> permutationList, ArrayList<Integer> temp, int[] arr) {
        if (temp.size() == arr.length) {
            List<Integer> tempList = new ArrayList<>();
            for (int i : temp) {
                tempList.add(arr[i]);
            }
            permutationList.add(new ArrayList(tempList));
        } else {
            for (int i = 0; i < arr.length; i++) {
                if (!temp.contains(i)) {
                    temp.add(i);
                    generatePermutation(permutationList, temp, arr);
                    temp.remove(temp.size() - 1);
                }
            }
        }
    }
    private static void generateSubSequence(List<List<Integer>> subsequence, ArrayList<Integer> temp, int[] arr, int start) {
        subsequence.add(new ArrayList(temp));
        for (int i = start; i < arr.length; i++) {
            temp.add(arr[i]);
            generateSubSequence(subsequence, temp, arr, i + 1);
            temp.remove(temp.size() - 1);
        }
    }


}