LeetCode_ 491_ Longest increasing subsequence

Topic link

• https://leetcode.cn/problems/increasing-subsequences/

Title Description

Give you an array of integers nums , Find and return all different increasing subsequences in the array , In increasing subsequence There are at least two elements . You can press In any order Return to the answer .

The array may contain duplicate elements , If two integers are equal , It can also be regarded as 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]]

Tips ：

• 1 <= nums.length <= 15
• -100 <= nums[i] <= 100

Their thinking

backtracking , By way of example 1 As an example

The retrospective trilogy

• Parameters of recursive functions
  • startIndex：for Loop traversal
  • path： The results of the record meet the conditions of
  • ans： Set of recorded results
• Termination condition of recursive function
  • path.size >= 2
• Single layer logic
  • Here we need to pay attention to weight removal , That is, as shown in the figure , The elements used on the same layer under the same parent node can no longer be used

AC Code

class Solution {
    
    public List<List<Integer>> findSubsequences(int[] nums) {
    
        List<Integer> path = new ArrayList<>();
        List<List<Integer>> ans = new ArrayList<>();
        if (nums.length == 0) {
    
            return ans;
        }
        backTracing(nums, 0, path, ans);
        return ans;
    }

    private static void backTracing(int[] nums, int startIndex, List<Integer> path, List<List<Integer>> ans){
    
        if (path.size() >= 2) {
    
            ans.add(new ArrayList<>(path));
        }
        int[] flag = new int[201];
        for (int i = startIndex; i < nums.length; i++) {
    
            if (path.size() != 0 && nums[i] < path.get(path.size() - 1) || flag[nums[i] + 100] == 1) {
    
                continue;
            }
            flag[nums[i] + 100] = 1;
            path.add(nums[i]);
            backTracing(nums, i + 1, path, ans);
            path.remove(path.size() - 1);
        }
    }
}