Longest Repeating Character Replacement

0424.Longest-Repeating-Character-Replacement/memo.md

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+# 424. Longest Repeating Character Replacement
+
+## step1
+sliding windowを使う。
+23mほどかかった。
+
+時間計算量はO(N)。ただし文字列に含まれるユニークな文字を定数とみなした場合。
+
+他の解法は思いつかない。Hashmapがヒントにあるがこれをどう使うのかわからない。
+
+## 他の人のコード
+
+https://github.com/olsen-blue/Arai60/pull/49#issuecomment-3649179920
+
+https://github.com/Exzrgs/LeetCode/pull/48
+
+hashmapの使い方はこうするのか
+
+https://github.com/potrue/leetcode/pull/65/changes
+
+maxを取り出す部分でheapを使った解法。
+
+heapの遅延評価: heapからの削除が難しいからとりあえずcounterだけ更新しておいて、heapから取り出したときの値は一致していなかったら捨てる
+
+実際にはheapの定数倍の計算量で遅くなるようだ。
+
+https://leetcode.com/problems/longest-repeating-character-replacement/solutions/8301734/optimal-solution-by-nthapa000-lejz/
+
+max_frequencyは更新されるときだけ計算しても良く、時間計算量が小さくなる。が、毎回計算した方がわかりやすいようにも思う。
+
+## step2

0424.Longest-Repeating-Character-Replacement/step1.py

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+class Solution:
+    def characterReplacement(self, s: str, k: int) -> int:
+        length_of_longest = 0
+
+        def update_longest(target_alphabet):
+            nonlocal length_of_longest
+            left = 0
+            num_other_than_target = 0
+            for right in range(len(s)):
+                if s[right] != target_alphabet:
+                    num_other_than_target += 1
+                while num_other_than_target > k:
+                    if s[left] != target_alphabet:
+                        num_other_than_target -= 1
+                    left += 1
+                length_of_longest = max(length_of_longest, right - left + 1)
+
+        for target_alphabet in set(s):
+            update_longest(target_alphabet)
+
+        return length_of_longest

0424.Longest-Repeating-Character-Replacement/step2.py

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+import collections
+
+
+class Solution:
+    def characterReplacement(self, s: str, k: int) -> int:
+        count = collections.defaultdict(int)
+        left = 0
+        length_of_longest = 0
+
+        for right in range(len(s)):
+            count[s[right]] += 1
+
+            while (right - left + 1) - max(count.values()) > k:
+                count[s[left]] -= 1
+                left += 1
+
+            length_of_longest = max(length_of_longest, right - left + 1)
+
+        return length_of_longest

0424.Longest-Repeating-Character-Replacement/step2_heap.py

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+import collections
+import heapq
+
+
+class Solution:
+    def characterReplacement(self, s: str, k: int) -> int:
+        count = collections.defaultdict(int)
+        left = 0
+        length_of_longest = 0
+        heap = []
+
+        def get_max_count():
+            while -heap[0][0] != count[heap[0][1]]:
+                heapq.heappop(heap)
+            return -heap[0][0]
+
+        for right in range(len(s)):
+            count[s[right]] += 1
+            heapq.heappush(heap, (-count[s[right]], s[right]))
+
+            while (right - left + 1) - get_max_count() > k:
+                count[s[left]] -= 1
+                heapq.heappush(heap, (-count[s[left]], s[left]))
+                left += 1
+
+            length_of_longest = max(length_of_longest, right - left + 1)
+
+        return length_of_longest

0424.Longest-Repeating-Character-Replacement/step2_heap_revised.py

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+import collections
+import heapq
+
+
+class Solution:
+    def characterReplacement(self, s: str, k: int) -> int:
+        count = collections.defaultdict(int)
+        left = 0
+        length_of_longest = 0
+        heap = []  # contains (count, alphabet) in max heap
+
+        def get_max_count():
+            while heap[0][0] != count[heap[0][1]]:
+                heapq.heappop_max(heap)
+            return heap[0][0]
+
+        for right in range(len(s)):
+            count[s[right]] += 1
+            heapq.heappush_max(heap, (count[s[right]], s[right]))
+
+            while (right - left + 1) - get_max_count() > k:
+                count[s[left]] -= 1
+                heapq.heappush_max(heap, (count[s[left]], s[left]))
+                left += 1
+
+            length_of_longest = max(length_of_longest, right - left + 1)
+
+        return length_of_longest

0424.Longest-Repeating-Character-Replacement/step2_max_frequency.py

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+import collections
+
+
+class Solution:
+    def characterReplacement(self, s: str, k: int) -> int:
+        count = collections.defaultdict(int)
+        max_frequency = 0
+        left = 0
+        length_of_longest = 0
+
+        for right in range(len(s)):
+            count[s[right]] += 1
+            max_frequency = max(max_frequency, count[s[right]])
+
+            while (right - left + 1) - max_frequency > k:
+                count[s[left]] -= 1
+                left += 1
+
+            length_of_longest = max(length_of_longest, right - left + 1)
+
+        return length_of_longest