Leetcode-208. 实现 Trie (前缀树)

实现一个 Trie (前缀树)，包含 insert, search, 和 startsWith 这三个操作。

示例:

Trie trie = new Trie();

trie.insert("apple");
trie.search("apple");   // 返回 true
trie.search("app");     // 返回 false
trie.startsWith("app"); // 返回 true
trie.insert("app");   
trie.search("app");     // 返回 true

说明:

• 你可以假设所有的输入都是由小写字母 a-z 构成的。
• 保证所有输入均为非空字符串。

解法：其实Trie的实现与链表有一些类似，保持root节点不变，对后面的分支进行添加，有26个英文字符，所以选择26长度的数组，如果是256的ASCII码则使用HashMap即可，

• Java

class TrieNode {
   
    public char val;
    public TrieNode[] children = new TrieNode[26];
    public boolean isWord;
    public TrieNode(char c) {
   
        this.val = c;
    }
}
class Trie {
   
    public TrieNode root = new TrieNode(' ');
    /** Initialize your data structure here. */
    public Trie() {
   }
    
    /** Inserts a word into the trie. */
    public void insert(String word) {
   
        TrieNode tmp = root; // 类似链表中头结点不改变，采用指针指向头结点
        for (int i=0;i<word.length();i++) {
   
            char c = word.charAt(i);
            if (tmp.children[c-'a']==null) 
                tmp.children[c-'a'] = new TrieNode(c);
            tmp = tmp.children[c-'a'];
        }
        tmp.isWord = true;
    }
    
    /** Returns if the word is in the trie. */
    public boolean search(String word) {
   
        TrieNode tmp = root;
        for (int i=0;i<word.length();i++) {
   
            char c = word.charAt(i);
            if (tmp.children[c-'a']==null)
                return false;
            tmp = tmp.children[c-'a'];
        }
        return tmp.isWord;
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    public boolean startsWith(String prefix) {
   
        TrieNode tmp = root;
        for (int i=0;i<prefix.length();i++) {
   
            char c = prefix.charAt(i);
            if (tmp.children[c-'a']==null)
                return false;
            tmp = tmp.children[c-'a'];
        }
        return true;
    }
}

/** * Your Trie object will be instantiated and called as such: * Trie obj = new Trie(); * obj.insert(word); * boolean param_2 = obj.search(word); * boolean param_3 = obj.startsWith(prefix); */

• Python

class TrieNode:
    def __init__(self,val):
        self.val = val
        self.children = [None]*26
        self.isWord = False
class Trie:

    def __init__(self):
        """ Initialize your data structure here. """
        self.root = TrieNode('')

    def insert(self, word: str) -> None:
        """ Inserts a word into the trie. """
        tmp = self.root
        for c in word:
            if tmp.children[ord(c)-ord('a')] == None:
                tmp.children[ord(c)-ord('a')] = TrieNode(c);
            tmp = tmp.children[ord(c)-ord('a')]
        tmp.isWord = True
                

    def search(self, word: str) -> bool:
        """ Returns if the word is in the trie. """
        tmp = self.root
        for c in word:
            if tmp.children[ord(c)-ord('a')] == None: 
                return False
            tmp = tmp.children[ord(c)-ord('a')]
        return tmp.isWord
        

    def startsWith(self, prefix: str) -> bool:
        """ Returns if there is any word in the trie that starts with the given prefix. """
        tmp = self.root
        for c in prefix:
            if tmp.children[ord(c)-ord('a')] == None:
                return False
            tmp = tmp.children[ord(c)-ord('a')]
        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)

附上Python语法糖的写法，使用map

class Trie:

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

    def insert(self, word: str) -> None:
        """ Inserts a word into the trie. """
        node = self.root
        for char in word:
            node = node.setdefault(char,{
   })
        node[self.isWord] = self.isWord
        

    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.isWord in node
        

    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)