Leetcode 208: Implement Trie (Prefix Tree)

Implement Trie (Prefix Tree)

Total Accepted: 601 Total Submissions: 2396

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

Note:
You may assume that all inputs are consist of lowercase letters a-z.

[思路]

用hashmap 存children, 用一个bool变量标示是否有一个word到此node结束.

[CODE]

class TrieNode {
    // Initialize your data structure here.
    char c;
    boolean leaf;
    HashMap<Character, TrieNode> children = new HashMap<Character, TrieNode>();
    
    public TrieNode(char c) {
        this.c = c;
    }
        
    public TrieNode(){};
}

public class Trie {
    private TrieNode root;

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

    // Inserts a word into the trie.
    public void insert(String word) {
        Map<Character, TrieNode> children = root.children;
        for(int i=0; i<word.length(); i++) {
            char c = word.charAt(i);
            TrieNode t;
            if(children.containsKey(c)) {
                t = children.get(c);
            } else {
                t = new TrieNode(c);
                children.put(c, t);
            }
            children = t.children;
            if(i==word.length()-1) t.leaf=true;
        }
    }

    // Returns if the word is in the trie.
    public boolean search(String word) {
        TrieNode t = searchNode(word);
        return t!=null && t.leaf;
    }

    // Returns if there is any word in the trie
    // that starts with the given prefix.
    public boolean startsWith(String prefix) {
        return searchNode(prefix) != null;
    }
    
    private TrieNode searchNode(String word) {
        Map<Character, TrieNode> children = root.children;
        TrieNode t = null;
        for(int i=0; i<word.length(); i++) {
            char c = word.charAt(i);
            if(!children.containsKey(c)) return null;
            t = children.get(c);
            children = t.children;
        }
        return t;
    }
}

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