leetcode解题: Implement Trie (Prefix Tree) (208)

Implement a trie with insert, search, and startsWith methods.

解法1：

本题主要是考察Trie树的基本概念，可以参考Wiki的解释。 实现的时候，每一个node的children可以用一个map来实现，做insert或者是search的时候可以用一个current的指针从root开始不停的往下搜索。
比较直观，并没有难度。
C++

class TrieNode {
public:
    // Initialize your data structure here.
    TrieNode() {
        isWord = false;
    }
    
    unordered_map<char, TrieNode*> children;
    bool isWord;
};

class Trie {
public:
    Trie() {
        root = new TrieNode();
    }

    // Inserts a word into the trie.
    void insert(string word) {
        TrieNode* cur = root;
        for (char c: word) {
            if (!cur->children.count(c)) {
                cur->children[c] = new TrieNode();
            }
            cur = cur->children[c];
        }
        cur->isWord = true;
    }

    // Returns if the word is in the trie.
    bool search(string word) {
        TrieNode* cur = root;
        for (char c: word) {
            if (!cur->children.count(c)) {
                return false;
            }
            cur = cur->children[c];
        }
        return cur->isWord;
    }

    // Returns if there is any word in the trie
    // that starts with the given prefix.
    bool startsWith(string prefix) {
        TrieNode* cur = root;
        for (char c: prefix) {
            if (!cur->children.count(c)) {
                return false;
            }
            cur = cur->children[c];
        }
        return true;
    }

private:
    TrieNode* root;
};

// Your Trie object will be instantiated and called as such:
// Trie trie;
// trie.insert("somestring");
// trie.search("key");

Java

class TrieNode {
    Map<Character, TrieNode> map;
    boolean isWord;
    public TrieNode() {
        this.map = new HashMap<Character, TrieNode>();
    }
};

public class Trie {
    private TrieNode root;
    /** Initialize your data structure here. */
    public Trie() {
        this.root = new TrieNode();
    }
    
    /** Inserts a word into the trie. */
    public void insert(String word) {
        TrieNode current = root;
        for (char c : word.toCharArray()) {
            if (!current.map.containsKey(c)) {
                current.map.put(c, new TrieNode());
            }
            current = current.map.get(c);
        }
        current.isWord = true;
    }
    
    /** Returns if the word is in the trie. */
    public boolean search(String word) {
        TrieNode current = root;
        for (char c : word.toCharArray()) {
            if (!current.map.containsKey(c)) {
                return false;
            }
            current = current.map.get(c);
        }
        return current.isWord;
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    public boolean startsWith(String prefix) {
        TrieNode current = root;
        for (char c: prefix.toCharArray()) {
            if (!current.map.containsKey(c)) {
                return false;
            }
            current = current.map.get(c);
        }
        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);
 */