125 lines
5.0 KiB
Markdown
125 lines
5.0 KiB
Markdown
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---
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title: Data Structure Trie
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localeTitle: 数据结构Trie
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---
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## 特里简介
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单词trie是单词“re **trie** val”的缩写，因为trie可以在字典中找到只有单词前缀的单个单词。
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Trie是一种高效的数据检索数据结构，使用trie，搜索复杂度可以达到最佳极限，即字符串的长度。
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当我们存储字符串时，它是一种用于在字母表上存储字符串的多路树结构。
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它已被用来存储大型英语词典，比如拼写检查程序中的单词。
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但是，尝试的惩罚是存储要求。
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## 什么是特里？
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trie是一种树状数据结构，它存储字符串，并帮助您使用字符串的前缀查找与该字符串关联的数据。
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例如，假设您计划构建字典以存储字符串及其含义。您一定想知道为什么我不能简单地使用哈希表来获取信息。
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是的，您显然可以使用哈希表获取信息，但是， 哈希表只能找到字符串与我们添加的字符串完全匹配的数据。但是，与哈希表相比，trie将使我们能够在更短的时间内找到具有公共前缀，缺少字符等的字符串。
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通常，特里看起来像这样，
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这是一个Trie的图像，它存储了单词{assoc，algo，all，also，tree，trie}。
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## 如何实现trie？
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让我们在python中实现一个trie，用于存储带有英语词典含义的单词。
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```
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ALPHABET_SIZE = 26 # For English
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class TrieNode:
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def __init__(self):
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self.edges = [None]*(ALPHABET_SIZE) # Each index respective to each character.
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self.meaning = None # Meaning of the word.
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self.ends_here = False # Tells us if the word ends here.
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```
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如您所见，边长为26，每个索引指的是字母表中的每个字符。 'A'对应0，'B'对应1，'C'对2 ......'Z'对应第25个索引。如果您要查找的字符指向`None` ，则表示该字符不在trie中。
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典型的Trie应该至少实现这两个功能：
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* `add_word(word,meaning)`
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* `search_word(word)`
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* `delete_word(word)`
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此外，还可以添加类似的东西
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* `get_all_words()`
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* `get_all_words_with_prefix(prefix)`
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#### 将Word添加到trie
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```
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def add_word(self,word,meaning):
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if len(word)==0:
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self.ends_here = True # Because we have reached the end of the word
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self.meaning = meaning # Adding the meaning to that node
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return
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ch = word[0] # First character
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# ASCII value of the first character (minus) the ASCII value of 'a'-> the first character of our ALPHABET gives us the index of the edge we have to look up.
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index = ord(ch) - ord('a')
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if self.edges[index] == None:
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# This implies that there's no prefix with this character yet.
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new_node = TrieNode()
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self.edges[index] = new_node
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self.edges[index].add(word[1:],meaning) #Adding the remaining word
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```
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#### 检索数据
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```
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def search_word(self,word):
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if len(word)==0:
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if self.ends_here:
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return True
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else:
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return "Word doesn't exist in the Trie"
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ch = word[0]
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index = ord(ch)-ord('a')
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if self.edge[index]== None:
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return False
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else:
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return self.edge[index].search_word(word[1:])
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```
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`search_word`函数将告诉我们Trie中是否存在该单词。由于我们的是字典，我们也需要获取含义，现在让我们声明一个函数来做到这一点。
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```
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def get_meaning(self,word):
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if len(word)==0 :
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if self.ends_here:
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return self.meaning
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else:
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return "Word doesn't exist in the Trie"
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ch = word[0]
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index = ord(ch) - ord('a')
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if self.edges[index] == None:
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return "Word doesn't exist in the Trie"
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else:
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return self.edges[index].get_meaning(word[1:])
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```
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#### 删除数据
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通过删除数据，您只需将变量`ends_here`更改为`False` 。这样做不会改变前缀，但是仍然会删除trie中单词的含义和存在。
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```
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def delete_word(self,word):
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if len(word)==0:
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if self.ends_here:
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self.ends_here = False
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self.meaning = None
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return "Deleted"
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else:
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return "Word doesn't exist in the Trie"
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ch = word[0]
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index = ord(ch) - ord('a')
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if self.edges[index] == None:
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return "Word doesn't exist in the Trie"
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else:
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return self.edges[index].delete_word(word[1:])
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```
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 [运行代码](https://repl.it/CWbr)
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## 资源
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* 如需进一步阅读，您可以试试这个[topcoder](https://www.topcoder.com/community/data-science/data-science-tutorials/using-tries/)教程。
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* 另外， [geeksforgeeks](http://www.geeksforgeeks.org/trie-insert-and-search/)的教程
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