diff --git a/include/s0491_non_decreasing_subsequences.hpp b/include/s0491_non_decreasing_subsequences.hpp
new file mode 100644
index 0000000..19e0821
--- /dev/null
+++ b/include/s0491_non_decreasing_subsequences.hpp
@@ -0,0 +1,14 @@
+#ifndef S0491_NON_DECREASING_SUBSEQUENCES_HPP
+#define S0491_NON_DECREASING_SUBSEQUENCES_HPP
+
+#include <unordered_map>
+#include <vector>
+
+using namespace std;
+
+class S0491 {
+ public:
+  vector<vector<int>> findSubsequences(vector<int>& nums);
+};
+
+#endif
diff --git a/notes/src/subsets.md b/notes/src/subsets.md
index 4eaa8ac..19db122 100644
--- a/notes/src/subsets.md
+++ b/notes/src/subsets.md
@@ -3,3 +3,42 @@
 ## [78. 子集](https://leetcode.cn/problems/subsets/)
 
 其实和切割问题非常像。
+
+## [90. 子集 II](https://leetcode.cn/problems/subsets-ii/)
+
+就是在 s0078 的基础上加了去重逻辑，和组合问题中的去重逻辑一样。
+
+## [491. 递增子序列](https://leetcode.cn/problems/non-decreasing-subsequences/)
+
+也是要去重，但是不能通过排序去重，因为排序之后顺序就全部打乱了。
+
+这个去重同样要保留树枝重复，但去除树层重复。
+
+思路很简单，每一层遍历的时候创建一个哈希表，用来记录当前元素是否遍历过，如果遍历过则跳过。
+
+```cpp
+void findSubsequencesDFS(vector<int> &subsequences, vector<vector<int>> &result,
+                         vector<int> &nums, int startIndex) {
+  int size = nums.size();
+  // 结束条件
+  if (startIndex == size) return;
+
+  // 初始化一个哈希表用来存储元素是否在数层中使用过
+  unordered_map<int, bool> used;
+
+  for (int i = startIndex; i < size; ++i) {
+    // 剪枝，如果元素在数层中使用过则跳过
+    if (used.count(nums[i]) == 1) continue;
+    // 当当前元素大于等于起始元素之前的元素时，将它添加进去
+    if (startIndex == 0 || nums[i] >= nums[startIndex - 1]) {
+      subsequences.push_back(nums[i]);
+      if (subsequences.size() > 1) result.push_back(subsequences);
+      used[nums[i]] = true;
+      findSubsequencesDFS(subsequences, result, nums, i + 1);
+      subsequences.pop_back();
+    }
+  }
+}
+```
+
+这种去重逻辑相比于之前的去重逻辑起始开销更大，因为每一层遍历都要创建一个新的哈希表，而之前的去重逻辑每一层遍历都用的是之前创建的向量。
diff --git a/src/s0491_non_decreasing_subsequences.cpp b/src/s0491_non_decreasing_subsequences.cpp
new file mode 100644
index 0000000..f83711e
--- /dev/null
+++ b/src/s0491_non_decreasing_subsequences.cpp
@@ -0,0 +1,31 @@
+#include "s0491_non_decreasing_subsequences.hpp"
+
+void findSubsequencesDFS(vector<int> &subsequences, vector<vector<int>> &result,
+                         vector<int> &nums, int startIndex) {
+  int size = nums.size();
+  // 结束条件
+  if (startIndex == size) return;
+
+  // 初始化一个哈希表用来存储元素是否在数层中使用过
+  unordered_map<int, bool> used;
+
+  for (int i = startIndex; i < size; ++i) {
+    // 剪枝，如果元素在数层中使用过则跳过
+    if (used.count(nums[i]) == 1) continue;
+    // 当当前元素大于等于起始元素之前的元素时，将它添加进去
+    if (startIndex == 0 || nums[i] >= nums[startIndex - 1]) {
+      subsequences.push_back(nums[i]);
+      if (subsequences.size() > 1) result.push_back(subsequences);
+      used[nums[i]] = true;
+      findSubsequencesDFS(subsequences, result, nums, i + 1);
+      subsequences.pop_back();
+    }
+  }
+}
+
+vector<vector<int>> S0491::findSubsequences(vector<int> &nums) {
+  vector<int> subsequences{};
+  vector<vector<int>> result{};
+  findSubsequencesDFS(subsequences, result, nums, 0);
+  return result;
+}