Question

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

Analysis

递归回溯（DFS)找到所有的递增子序列，使用set来自动去除重复的项。

Solution

class Solution {
    public List<List<Integer>> findSubsequences(int[] nums) {
        Set<List<Integer>> res = new HashSet<>();
        List<Integer> holder = new ArrayList<>();
        findSubsequence(res,holder,0,nums);
        List ans = new ArrayList(res);
        return ans;
    }
    private void findSubsequence(Set<List<Integer>> res,List<Integer>  holder,int index, int[] nums){
        if(holder.size() >= 2){
            res.add(new ArrayList(holder));
        }
        for(int i = index;i<nums.length;i++){
            if(holder.size()==0 || holder.get(holder.size()-1) <= nums[i]){
                holder.add(nums[i]);
                findSubsequence(res,holder,i+1,nums);
                holder.remove(holder.size()-1);
            }
        }
    }
}