Given an integer array, your task is to find all the different possible increasing subsequences of the given array, and the length of an increasing subsequence should be at least 2 .

Example:

Input: [4, 6, 7, 7]

Output: [[4, 6], [4, 7], [4, 6, 7], [4, 6, 7, 7], [6, 7], [6, 7, 7], [7,7], [4,7,7]]

Solution: 每层用hashset去重

    public List<List<Integer>> findSubsequences(int[] nums) {
        List<List<Integer>> ans = new ArrayList<>();
        if (nums == null || nums.length == 0) {
            return ans;
        }
        helper(ans, new ArrayList<Integer>(), 0, nums);
        return ans;
    }
    private void helper(List<List<Integer>> ans, List<Integer> path, int index, int[] nums) {
        if (path.size() > 1) {
            ans.add(new ArrayList<>(path));
        }
        Set<Integer> hashset = new HashSet<>();
        for (int i = index; i < nums.length; i++) {
            if (!hashset.contains(nums[i]) && (path.size() == 0 || nums[i] >= path.get(path.size() - 1))) {
                hashset.add(nums[i]);
                path.add(nums[i]);
                //注意是i + 1，不是index + 1
                helper(ans, path, i + 1, nums);
                path.remove(path.size() - 1);
            }
        }
    }