139. Word Break

139_word_break/problem.md

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+## 問題: [139. Word Break](https://leetcode.com/problems/word-break/description/)

139_word_break/step1.md

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+# Step 1
+
+- わからなかったので[NeetCodeの解説](https://www.youtube.com/watch?v=Sx9NNgInc3A)を見た
+  - 末尾から見て「ここから先を辞書の語で切れるか」を`boolean_list[i]`に記録するDP
+  - 空文字は `True`（`boolean_list[len(s)]`）。各位置 `i` で、辞書の語が `s` の先頭（`i` から）に合えば、その語の直後 `i+len(word)` が `True` なら `i` も `True`。最後に `boolean_list[0]` が答え。
+
+```python
+class Solution:
+    def wordBreak(self, s: str, wordDict: List[str]) -> bool:
+        boolean_list = [False] * (len(s) + 1)
+        boolean_list[len(s)] = True
+
+        for i in range(len(s) - 1, -1, -1):
+            for word in wordDict:
+                if i + len(word) <= len(s) and s[i:i + len(word)] in wordDict:
+                    boolean_list[i] = boolean_list[i + len(word)]
+                if boolean_list[i]:
+                    break
+        return boolean_list[0]
+```
+
+時間計算量: $O(nmt)$
+
+空間計算量: $O(n)$

139_word_break/step2.md

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+# Step 2
+
+- DPテーブルの変数、良い名付け方が思いつかない
+
+```python
+class Solution:
+    def wordBreak(self, s: str, wordDict: List[str]) -> bool:
+        boolean_list = [False] * (len(s) + 1)
+        boolean_list[len(s)] = True
+
+        for i in range(len(s) - 1, -1, -1):
+            for word in wordDict:
+                if i + len(word) <= len(s) and s[i:i + len(word)] in wordDict:
+                    boolean_list[i] = boolean_list[i + len(word)]
+                if boolean_list[i]:
+                    break
+        return boolean_list[0]
+```
+
+時間計算量: $O(nmt)$
+
+空間計算量: $O(n)$

139_word_break/step3.md

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+# Step 3
+
+```python
+class Solution:
+    def wordBreak(self, s: str, wordDict: List[str]) -> bool:
+        boolean_list = [False] * (len(s) + 1)
+        boolean_list[len(s)] = True
+
+        for i in range(len(s) - 1, -1, -1):
+            for word in wordDict:
+                if i + len(word) <= len(s) and s[i:i + len(word)] in wordDict:
+                    boolean_list[i] = boolean_list[i + len(word)]
+                if boolean_list[i]:
+                    break
+        return boolean_list[0]
+```
+
+1回目: 2分　18秒
+
+2回目: 2分　9秒
+
+3回目: 1分　54秒