347. Top K Frequent Elements

347_top-k-frequent-elements.md

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+# 347. Top K Frequent Elements
+
+## 1st
+
+所要時間: 3:19
+
+n: len(nums), m: unique(nums)
+- 時間計算量: O(n + k*log(m)) (most_common()はnがNoneでなければheapq.nlargest()を実行する [コード](https://github.com/python/cpython/blob/3.12/Lib/collections/__init__.py#L618))
+- 空間計算量: O(m)
+
+Counterを知っていたのですぐ解けた。 `[num for num, _ in tally.most_common(k)][:k]` の方がいいか
+
+```python
+class Solution:
+    def topKFrequent(self, nums: List[int], k: int) -> List[int]:
+        tally = Counter(nums)
+        return list(map(lambda t: t[0], tally.most_common(k)))
+```
+
+---
+
+defaultdictを使った。何となくdataclassを使う。どうせならfrozen=Trueにしても良かったか。
+`__lt__` だけの実装にしたが、確認したところ[pep8](https://peps.python.org/pep-0008/#programming-recommendations)では推奨していないらしい。
+
+> When implementing ordering operations with rich comparisons, it is best to implement all six operations (__eq__, __ne__, __lt__, __le__, __gt__, __ge__) rather than relying on other code to only exercise a particular comparison.
+> To minimize the effort involved, the functools.total_ordering() decorator provides a tool to generate missing comparison methods.
+
+一方 functools.total_ordering() も速度やスタックトレースが複雑になるというデメリットがある模様。 https://docs.python.org/3/library/functools.html#functools.total_ordering
+
+> While this decorator makes it easy to create well behaved totally ordered types, it does come at the cost of slower execution and more complex stack traces for the derived comparison methods. If performance benchmarking indicates this is a bottleneck for a given application, implementing all six rich comparison methods instead is likely to provide an easy speed boost.
+
+
+所要時間: 8:11
+
+n: len(nums), m: unique(nums)
+- 時間計算量: O(n + mlog(m))
+- 空間計算量: O(m)
+
+```python
+from dataclasses import dataclass
+
+@dataclass
+class NumWithFreq:
+    num: int
+    freq: int
+
+    def __lt__(self, other):
+        return self.freq < other.freq
+
+class Solution:
+    def topKFrequent(self, nums: List[int], k: int) -> List[int]:
+        tally = defaultdict(int)
+        for num in nums:
+            tally[num] += 1
+        return list(map(
+            lambda num_with_freq: num_with_freq.num,
+            sorted(map(lambda t: NumWithFreq(*t), tally.items()), reverse=True)[:k]
+        ))
+```
+
+---
+quick selectによる解法
+
+所要時間: 49:15
+
+バグが全く取れず苦戦した。namedtupleで定義したNumWithFreqがコードに残っており、そのせいで比較がおかしくなっていた。
+
+n: len(nums), m: unique(nums)
+- 時間計算量: 平均O(m)
+- 空間計算量: O(m)
+
+```python
+from dataclasses import dataclass
+
+@total_ordering
+@dataclass(frozen=True)
+class NumWithFreq:
+    num: int
+    freq: int
+
+    def __lt__(self, other):
+        return self.freq < other.freq
+
+    def __eq__(self, other):
+        return self.freq == other.freq
+
+
+class Solution:
+    def topKFrequent(self, nums: List[int], k: int) -> List[int]:
+        assert len(nums) > 0
+        tally = defaultdict(int)
+        for num in nums:
+            tally[num] += 1
+        num_with_freqs = list(map(lambda t: NumWithFreq(*t), tally.items()))
+        left = 0
+        right = len(num_with_freqs) - 1
+        while left <= right:
+            pivot = self._partition(num_with_freqs, left, right)
+            if pivot == k - 1:
+                return list(map(lambda e: e.num, num_with_freqs))[:k]
+            if pivot < k - 1:
+                left = pivot + 1
+            else:
+                right = pivot - 1
+        return [] # never reached
+
+    def _partition(self, values: List[NumWithFreq], left: int, right: int) -> int:
+        pivot = self._getPivot(values, left, right)
+        values[pivot], values[right] = values[right], values[pivot]
+        i = left - 1
+        for j in range(left, right):
+            # descendent order
+            if values[j] > values[right]:
+                i += 1
+                values[i], values[j] = values[j], values[i]
+        values[i+1], values[right] = values[right], values[i+1]
+        return i + 1
+
+    # 3点の真ん中の値を返す
+    def _getPivot(self, values: List[NumWithFreq], left: int, right: int) -> int:
+        mid = (left + right) // 2
+        if values[left] <= values[mid]:
+            if values[mid] <= values[right]:
+                return mid
+            elif values[left] <= values[right]:
+                return right
+            return left
+        # case of values[mid] < values[left]
+        if values[left] <= values[right]:
+            return left
+        elif values[mid] <= values[right]:
+            return right
+        return mid
+```
+
+
+## 2nd
+
+### 参考
+
+- https://discord.com/channels/1084280443945353267/1231966485610758196/1243547482169016341
+- https://github.com/cheeseNA/leetcode/pull/13
+- https://github.com/hayashi-ay/leetcode/pull/60
+
+略
+
+## 3rd
+
+略