python 字典树、前缀匹配

在python中可以用字典来实现字典树这一结构。

class Trie:

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.root = {}
        self.isEnd = '#'

    def insert(self, word: str) -> None:
        """
        Inserts a word into the trie.
        """
        node = self.root
        for char in word:
            if char not in node:
                node[char] = {}  #如果没有这一字母则新建一个字典
            node = node[char]
        node[self.isEnd] = '#'   #加上结束符

    def search(self, word: str) -> bool:
        """
        Returns if the word is in the trie.
        """
        node = self.root
        for char in word:
            if char not in node:
                return False
            node = node[char]
        return self.isEnd in node  #如果没有# 说明只是前缀


    def startsWith(self, prefix: str) -> bool:
        """
        Returns if there is any word in the trie that starts with the given prefix.
        """
        node = self.root
        for char in prefix:
            if char not in node:
                return False
            node = node[char]
        return True



# Your Trie object will be instantiated and called as such:
# obj = Trie()
# obj.insert(word)
# param_2 = obj.search(word)
# param_3 = obj.startsWith(prefix)

2.采用defaultdict创建trie

from collections import defaultdict
from functools import reduce
TrieNode = lambda: defaultdict(TrieNode)
class Trie:
    def __init__(self):
        self.trie = TrieNode()
    def insert(self, word):
        reduce(dict.__getitem__, word, self.trie)['end'] = True
    def search(self, word):
        return reduce(lambda d,k: d[k] if k in d else TrieNode(), word, self.trie).get('end', False)
    def startsWith(self, word):
        return bool(reduce(lambda d,k: d[k] if k in d else TrieNode(), word, self.trie).keys())

collections，解释是数据类型容器模块。这里面有一个collections.defaultdict()经常被用到

这里的defaultdict(function_factory)构建的是一个类似dictionary的对象，其中keys的值，自行确定赋值，但是values的类型，是function_factory的类实例，而且具有默认值。比如default(int)则创建一个类似dictionary对象，里面任何的values都是int的实例，而且就算是一个不存在的key, d[key] 也有一个默认值，这个默认值是int()的默认值0.