208. Implement Trie (Prefix Tree)

Question

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

Example:

Trie trie = new Trie();

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

Note:

    You may assume that all inputs are consist of lowercase letters a-z.
    All inputs are guaranteed to be non-empty strings.

Thinking:

• Method：
  • 使用内部类TrieNode.

class Trie {
    TrieNode root;
    /** Initialize your data structure here. */
    public Trie() {
        root = new TrieNode();
    }
    /** Inserts a word into the trie. */
    public void insert(String word) {
        TrieNode temp = root;
        for(int i = 0; i < word.length(); i++){
            int c = word.charAt(i) - 'a';
            if(temp.childs[c] == null)
                temp.childs[c] = new TrieNode();
            temp = temp.childs[c];
        }
        temp.isLeaf = true;
    }
    /** Returns if the word is in the trie. */
    public boolean search(String word) {
        TrieNode temp = root;
        for(int i = 0; i < word.length(); i++){
            int c = word.charAt(i) - 'a';
            if(temp.childs[c] != null) temp = temp.childs[c];
            else return false;
        }
        return temp.isLeaf;
    }
    /** Returns if there is any word in the trie that starts with the given prefix. */
    public boolean startsWith(String prefix) {
        TrieNode temp = root;
        for(int i = 0; i < prefix.length(); i++){
            int c = prefix.charAt(i) - 'a';
            if(temp.childs[c] == null) return false;
            else temp = temp.childs[c];
        }
        return true;
    }
    private static class TrieNode{
        boolean isLeaf;
        TrieNode[] childs;
        TrieNode(){
            childs = new TrieNode[26];
        }
    }
}

/**
 * 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);
 */

二刷

1. 最典型的字典树，将每个字符作为一个结点，我们通过遍历的方式，将一个字符串的所有字符添加进入当前的树中。
2. 如果已经到了最后一个字符，我们将当前节点的isLeaf设置成true。

class Trie {
    private static class TrieNode{
        boolean isLeaf;
        TrieNode[] childs;
        public TrieNode(){
            childs = new TrieNode[26];
        }
    }
    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) {
        int len = word.length();
        TrieNode temp = root;
        for(int i = 0; i < len; i++){
            int c = word.charAt(i) - 'a';
            if(temp.childs[c] == null){
                temp.childs[c] = new TrieNode();
            }
            temp = temp.childs[c];
        }
        temp.isLeaf = true;
    }
    
    /** Returns if the word is in the trie. */
    public boolean search(String word) {
        if(word == null || word.length() == 0) return true;
        int len = word.length();
        TrieNode temp = this.root;
        for(int i = 0; i < len; i++){
            int c = word.charAt(i) - 'a';
            if(temp.childs[c] == null) return false;
            else temp = temp.childs[c];
        }
        return temp.isLeaf;
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    public boolean startsWith(String prefix) {
        if(prefix == null || prefix.length() == 0) return false;
        int len = prefix.length();
        TrieNode temp = root;
        for(int i = 0; i < len; i++){
            int c = prefix.charAt(i) - 'a';
            if(temp.childs[c] == null) return false;
            temp = temp.childs[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);
 */

Reference

1. Trie Tree 字典树