題目

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:

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

題目大意

實現一個Trie，其中 insert 表示 插入字串 search 表示 查詢字串 startWith 表示 是否有該字串 作為 字首

思路

內建類 node 作為樹的結點。

node 指向 26個node，同時 Boolean isLeaf 標註是 該結點是否為 字串的尾部。

class Trie {

    class Node {
 

        Node[] childs = new Node[26];
        boolean isLeaf;
    }
    private Node root = new Node();

    /** Initialize your data structure here. */
    public Trie() {

    }

    /** Inserts a word into the trie. */
    public void insert(String word) {
        insert(word,root);
    }

    private
 
 void insert(String word, Node node) {
        if(root == null)
            return;
        if(word.length() == 0){
            node.isLeaf = true;
            return;
        }     
        int index = word.charAt(0) - 'a';
        if(node.childs[index] == null)
            node.childs[index] = new Node();
        insert(word.substring(1),node.childs[index]);
    }
    /** Returns if the word is in the trie. */
    public boolean search(String word) {
        return search(word,root);
    }

    private boolean search(String word, Node node) {
        if(node == null)
            return false;
        if(word.length() == 0)
            return node.isLeaf;
        int index = word.charAt(0) - 'a';
        return search(word.substring(1),node.childs[index]);
    }

    /** Returns if there is any word in the trie that starts with the given prefix. */
    public boolean startsWith(String prefix) {
        return startsWith(prefix,root);
    }

    private boolean startsWith(String prefix, Node node) {
        if(node == null)
            return false;
        if(prefix.length() == 0)
            return true;
        int index = prefix.charAt(0) - 'a';
        return startsWith(prefix.substring(1),node.childs[index]);
    }
}
/**
 * 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);
 */