问题：

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]]

Note:

1. The length of the given array will not exceed 15.
2. The range of integer in the given array is [-100,100].
3. The given array may contain duplicates, and two equal integers should also be considered as a special case of increasing sequence.

解决：

① 递归解决。

class Solution {//62ms
    public List<List<Integer>> findSubsequences(int[] nums) {
        Set<List<Integer>> set = new HashSet<>();//避免重复
        List<Integer> list = new ArrayList<>();
        dfs(nums,0,list,set);
        return new ArrayList<>(set);
    }
    public void dfs(int[] nums,int i,List<Integer> list,Set<List<Integer>> set){
        if (list.size() >= 2){
            set.add(new ArrayList<>(list));
        }
        for (int j = i;j < nums.length;j ++){
            if (list.size() == 0 || list.get(list.size() - 1) <= nums[j]){
                list.add(nums[j]);
                dfs(nums,j + 1,list,set);
                list.remove(list.size() - 1);
            }
        }
    }
}

② 使用额外的标记记录重复。

class Solution {//25ms
    public List<List<Integer>> findSubsequences(int[] nums) {
        List<List<Integer>> res = new ArrayList<>();
        List<Integer> list = new ArrayList<>();
        dfs(nums,0,list,res);
        return res;
    }
    public void dfs(int[] nums,int i,List<Integer> list,List<List<Integer>> res){
        if (list.size() >= 2){
            res.add(new ArrayList<>(list));
        }
        if (i == nums.length) return;
        Set<Integer> isused = new HashSet<>(); 
        for (int j = i;j < nums.length;j ++){
            if (isused.contains(nums[j])) continue;
            if (list.isEmpty() || nums[j] >= list.get(list.size() - 1)){
                isused.add(nums[j]);
                list.add(nums[j]);
                dfs(nums,j + 1,list,res);
                list.remove(list.size() - 1);
            }
        }
    }
}