diff --git a/0090.Subsets-II/memo.md b/0090.Subsets-II/memo.md
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+++ b/0090.Subsets-II/memo.md
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+# 90. Subsets II
+
+## step1
+Arai 60の問題を覚えていたので、その応用で解けた。12mほど。
+
+時間、空間 O(n)
+
+Arai60の問題: https://leetcode.com/problems/subsets/
+
+もう少し簡潔にもできる
+
+~~ バックトラック ~~ 再帰 でも解く
+
+subsets[:] とすれば extention という変数を用意しなくてもかける
+
+## 他の人のコード
+https://leetcode.com/problems/subsets-ii/solutions/6877292/video-sorting-the-input-array-to-remove-wzqim/
+
+バックトラックはこの方法だ
+
+再帰をバックトラックで書いたつもりになっていた
+
+## step2
+バックトラック
+subsets.append(subset)として間違えた
+
+## step3
+バックトラック
diff --git a/0090.Subsets-II/step1_recursion.py b/0090.Subsets-II/step1_recursion.py
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+++ b/0090.Subsets-II/step1_recursion.py
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+import collections
+
+class Solution:
+    def subsetsWithDup(self, nums: list[int]) -> list[list[int]]:
+        def update_subsets(subsets, num, count):
+            extention = [subset + [num] * i for i in range(1, count + 1) for subset in subsets]
+            subsets.extend(extention)
+
+
+        num_to_count = collections.Counter(nums)
+        subsets = [[]]
+        for num, count in num_to_count.items():
+            update_subsets(subsets, num, count)
+
+        return subsets
+
+
diff --git a/0090.Subsets-II/step1_recursion_revised.py b/0090.Subsets-II/step1_recursion_revised.py
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index 0000000..1c37af2
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+++ b/0090.Subsets-II/step1_recursion_revised.py
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+import collections
+
+class Solution:
+    def subsetsWithDup(self, nums: list[int]) -> list[list[int]]:
+        def update_subsets(subsets, num, count):
+            subsets.extend([subset + [num] * i for i in range(1, count + 1) for subset in subsets[:]])
+
+
+        num_to_count = collections.Counter(nums)
+        subsets = [[]]
+        for num, count in num_to_count.items():
+            update_subsets(subsets, num, count)
+
+        return subsets
+
+
diff --git a/0090.Subsets-II/step1_subset.py b/0090.Subsets-II/step1_subset.py
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+++ b/0090.Subsets-II/step1_subset.py
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+import collections
+
+class Solution:
+    def subsetsWithDup(self, nums: list[int]) -> list[list[int]]:
+        num_to_count = collections.defaultdict(int)
+        for n in nums:
+            num_to_count[n] += 1
+
+        subsets = [[]]
+        for n, count in num_to_count.items():
+            extention = []
+            for i in range(1, count + 1):
+                extention.extend([subset + [n] * i for subset in subsets])
+            subsets.extend(extention)
+
+        return subsets
diff --git a/0090.Subsets-II/step1_subset_revised.py b/0090.Subsets-II/step1_subset_revised.py
new file mode 100644
index 0000000..fce7884
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+++ b/0090.Subsets-II/step1_subset_revised.py
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+import collections
+
+class Solution:
+    def subsetsWithDup(self, nums: list[int]) -> list[list[int]]:
+        num_to_count = collections.Counter(nums)
+        subsets = [[]]
+        for n, count in num_to_count.items():
+            subsets.extend([subset + [n] * i for i in range(1, count + 1) for subset in subsets])
+        return subsets
diff --git a/0090.Subsets-II/step2_backtrack.py b/0090.Subsets-II/step2_backtrack.py
new file mode 100644
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+++ b/0090.Subsets-II/step2_backtrack.py
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+class Solution:
+    def subsetsWithDup(self, nums: list[int]) -> list[list[int]]:
+        nums.sort()
+
+        def update_subsets(subsets, i, subset):
+            if i == len(nums):
+                subsets.append(subset[:])
+                return
+
+            subset.append(nums[i])
+            update_subsets(subsets, i+1, subset)
+            subset.pop()
+            while i + 1 < len(nums) and nums[i] == nums[i+1]:
+                i += 1
+            update_subsets(subsets, i+1, subset)
+
+        subsets = []
+        update_subsets(subsets, 0, [])
+
+        return subsets
+
+
diff --git a/0090.Subsets-II/step3.py b/0090.Subsets-II/step3.py
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+++ b/0090.Subsets-II/step3.py
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+class Solution:
+    def subsetsWithDup(self, nums: list[int]) -> list[list[int]]:
+        nums.sort()
+        subsets = []
+
+        subset = []
+        def update_subsets(i):
+            if i == len(nums):
+                subsets.append(subset[:])
+                return
+
+            subset.append(nums[i])
+            update_subsets(i+1)
+            subset.pop()
+            while i + 1 < len(nums) and nums[i] == nums[i+1]:
+                i += 1
+            update_subsets(i+1)
+
+        update_subsets(0)
+
+        return subsets
+
+