491. Increasing Subsequences

Problem

Given an integer array nums, return all the different possible increasing subsequences of the given array with at least two elements. You may return the answer in any order.

The given array may contain duplicates, and two equal integers should also be considered a special case of increasing sequence.

  Example 1:

Input: nums = [4,6,7,7]
Output: [[4,6],[4,6,7],[4,6,7,7],[4,7],[4,7,7],[6,7],[6,7,7],[7,7]]

Example 2:

Input: nums = [4,4,3,2,1]
Output: [[4,4]]

  Constraints:

• 1 <= nums.length <= 15

• -100 <= nums[i] <= 100

Solution (Java)

class Solution {
    public List<List<Integer>> findSubsequences(int[] nums) {
        if (nums == null || nums.length == 1) {
            return new ArrayList<>();
        }
        Set<List<Integer>> answer = new HashSet<>();
        List<Integer> list = new ArrayList<>();
        return new ArrayList<>(backtracking(nums, 0, list, answer));
    }

    private Set<List<Integer>> backtracking(
            int[] nums, int start, List<Integer> currList, Set<List<Integer>> answer) {
        if (currList.size() >= 2) {
            answer.add(new ArrayList<>(currList));
        }
        for (int i = start; i < nums.length; i++) {
            if (currList.isEmpty() || currList.get(currList.size() - 1) <= nums[i]) {
                currList.add(nums[i]);
                backtracking(nums, i + 1, currList, answer);
                currList.remove(currList.size() - 1);
            }
        }
        return answer;
    }
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).