diff --git a/ara60/top-k-frequent-elements/README.md b/ara60/top-k-frequent-elements/README.md
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+## 考察
+- 過去に解いたことあり
+- 方針
+    - まずはカウントを行う
+        - time: O(n), space: O(n)
+    - カウントした結果を元にtopkを算出
+        - 方針は3つ程思い浮かぶ
+        - ソート
+            - nums.length <= 10^5 なので、計数ソートが良さそう
+            - time: O(n), space: O(n)
+        - クイックセレクト
+            - ランダムにpivotを決める
+            - pivotの左側にpivot以下のもの、右側にpivotより大きいものを集める
+            - pivotが何番目かが確定するので、どちら側にk番目があるかは明白
+            - これを繰り返せばいい
+            - any orderなので、pivotがk番目に来たら、pivotとその左側を返せばいい
+            - time: O(n), space: O(1)
+        - ヒープを使う
+            - サイズkの最小ヒープを用意して、サイズを保つように入れていく
+            - time: O(n log k), space: O(k)
+- 全部さくっと実装できるようにしておきたい
+- まずはソートでやる
+- あとは実装
+
+## Step1
+- 計数ソートを実装した
+- time: O(n), space: O(n)
+
+## Step2
+- ヒープとクイックセレクトでも実装してみた
+- 3通り実装してみて、計数ソートとクイックセレクトは脳の負荷が高い
+- ヒープ使ってやるのが一番良さそう
+- あとC++、エラー内容がPythonよりわかりにくく感じた(これは慣れかもしれない)
+
+## Step3
+- 1回目: 3m02s
+- 2回目: 2m45s
+- 3回目: 2m30s
+
+## Step4
+- レビューを元に修正
+- 変数名の見直し
+    - num_count -> num_to_count
+    - min_heap -> min_count_and_nums
+    - key, value -> num, count
+- 拡張for文のコロンの前後にはspaceを入れる
+- pairの代わりにstructを使うように修正
+
diff --git a/ara60/top-k-frequent-elements/step1.cpp b/ara60/top-k-frequent-elements/step1.cpp
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+++ b/ara60/top-k-frequent-elements/step1.cpp
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+class Solution {
+public:
+    vector<int> topKFrequent(vector<int>& nums, int k) {
+        int n = nums.size();
+        unordered_map<int, int> num_count;
+        for (int num: nums) {
+            num_count[num]++;
+        }
+
+        vector<int> buckets(n + 1, 0);
+        for (auto [key, value]: num_count) {
+            buckets[value]++;
+        }
+        for (int i = 0; i < n; ++i) {
+            buckets[i + 1] += buckets[i];
+        }
+
+        vector<int> sorted(num_count.size());
+        for (auto [key, value]: num_count) {
+            sorted[--buckets[value]] = key;
+        }
+
+        return vector<int>(sorted.end() - k, sorted.end());
+    }
+};
diff --git a/ara60/top-k-frequent-elements/step2_heap.cpp b/ara60/top-k-frequent-elements/step2_heap.cpp
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+++ b/ara60/top-k-frequent-elements/step2_heap.cpp
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+class Solution {
+public:
+    vector<int> topKFrequent(vector<int>& nums, int k) {
+        unordered_map<int, int> count;
+        for (int num: nums) {
+            count[num]++;
+        }
+
+        priority_queue<pair<int, int>, vector<pair<int, int>>, greater<pair<int, int>>> min_heap;
+        for (auto [key, value]: count) {
+            min_heap.push({value, key});
+            if (min_heap.size() > k) {
+                min_heap.pop();
+            }
+        }
+
+        vector<int> answer;
+        while (!min_heap.empty()) {
+            answer.push_back(min_heap.top().second);
+            min_heap.pop();
+        }
+        return answer;
+    }
+};
diff --git a/ara60/top-k-frequent-elements/step2_quick_select.cpp b/ara60/top-k-frequent-elements/step2_quick_select.cpp
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+++ b/ara60/top-k-frequent-elements/step2_quick_select.cpp
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+class Solution {
+public:
+    vector<int> topKFrequent(vector<int>& nums, int k) {
+        unordered_map<int, int> count;
+        for (int num : nums) {
+            count[num]++;
+        }
+
+        vector<pair<int, int>> frequency_to_num;
+        for (auto& [key, value] : count) {
+            frequency_to_num.push_back({value, key});
+        }
+
+        int n = frequency_to_num.size();
+        quick_select(frequency_to_num, 0, n - 1, n - k);
+        vector<int> answer;
+        for (int i = n - 1; i >= n - k; --i) {
+            answer.push_back(frequency_to_num[i].second);
+        }
+        return answer;
+    }
+
+private:
+    void quick_select(vector<pair<int, int>>& array, int left, int right, int k) {
+        while (left < right) {
+            int pivot_index = partition(array, left, right);
+            if (pivot_index == k) return;
+            else if (pivot_index < k) left = pivot_index + 1;
+            else right = pivot_index - 1;
+        }
+    }
+
+    int partition(vector<pair<int, int>>& array, int left, int right) {
+        int pivot_index = rand_int(left, right);
+        int pivot_value = array[pivot_index].first;
+        swap(array[pivot_index], array[right]);
+        int store_index = left;
+        for (int i = left; i < right; ++i) {
+            if (array[i].first < pivot_value) {
+                swap(array[i], array[store_index]);
+                store_index++;
+            }
+        }
+        swap(array[store_index], array[right]);
+        return store_index;
+    }
+
+    int rand_int(int from, int to) {
+        return from + rand() % (to - from + 1);
+    }
+};
diff --git a/ara60/top-k-frequent-elements/step3.cpp b/ara60/top-k-frequent-elements/step3.cpp
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+++ b/ara60/top-k-frequent-elements/step3.cpp
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+class Solution {
+public:
+    vector<int> topKFrequent(vector<int>& nums, int k) {
+        unordered_map<int, int> num_count;
+        for (int num: nums) {
+            num_count[num]++;
+        }
+
+        priority_queue<pair<int, int>, vector<pair<int, int>>, greater<pair<int, int>>> min_heap;
+        for (auto [key, value]: num_count) {
+            min_heap.push({value, key});
+            if (min_heap.size() > k) {
+                min_heap.pop();
+            }
+        }
+
+        vector<int> answer;
+        while (!min_heap.empty()) {
+            answer.push_back(min_heap.top().second);
+            min_heap.pop();
+        }
+        return answer;
+    }
+};
diff --git a/ara60/top-k-frequent-elements/step4.cpp b/ara60/top-k-frequent-elements/step4.cpp
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+++ b/ara60/top-k-frequent-elements/step4.cpp
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+class Solution {
+public:
+    vector<int> topKFrequent(vector<int>& nums, int k) {
+        unordered_map<int, int> num_to_count;
+        for (int num : nums) {
+            num_to_count[num]++;
+        }
+
+        priority_queue<NumCount> min_count_and_nums;
+        for (auto [num, count] : num_to_count) {
+            min_count_and_nums.push({count, num});
+            if (min_count_and_nums.size() > k) {
+                min_count_and_nums.pop();
+            }
+        }
+
+        vector<int> answer;
+        while (!min_count_and_nums.empty()) {
+            answer.push_back(min_count_and_nums.top().num);
+            min_count_and_nums.pop();
+        }
+        return answer;
+    }
+
+private:
+    struct NumCount {
+        int count;
+        int num;
+
+        bool operator<(const NumCount& other) const {
+            return count > other.count;
+        }
+    };
+};