subject ：

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Trie（ It sounds like "try"） Or say Prefix tree It's a tree data structure , Keys for efficient storage and retrieval of string datasets . This data structure has quite a number of application scenarios , For example, automatic completion and spelling check .

Please realize Trie class ：

Trie() Initialize the prefix tree object .
void insert(String word) Insert a string into the forward tree word .
boolean search(String word) If the string word In the prefix tree , return true（ namely , Has been inserted before retrieval ）; otherwise , return false .
boolean startsWith(String prefix) If a string has been inserted before  word One of the prefixes of is prefix , return true ; otherwise , return false .

Answer key ：

Introduction to dictionary tree ：

startsWith and search Function has only the final judgment isEnd The difference between , So with searchPrefix The function implements the operation in front of the two functions .

class Trie {
    private Trie[] children;
    private boolean isEnd; // Whether the node is the end of a string 

    public Trie() {
        children = new Trie[26];
        isEnd = false;
    }
    
    public void insert(String word) {
        Trie node = this;
        for (int i = 0; i < word.length(); i++) {
            char ch = word.charAt(i);
            int index = ch - 'a';
            if (node.children[index] == null) {
                node.children[index] = new Trie();
            }
            node = node.children[index];
        }
        node.isEnd = true;
    }
    
    public boolean search(String word) {
        Trie node = searchPrefix(word);
        return node != null && node.isEnd;
    }
    
    public boolean startsWith(String prefix) {
        return searchPrefix(prefix) != null;
    }

    //startsWith and search Function has only the final judgment isEnd The difference between , So with searchPrefix The function implements the operation in front of the two functions 
    public Trie searchPrefix(String prefix) {
        Trie node = this;
        for (int i = 0; i < prefix.length(); i++) {
            char ch = prefix.charAt(i);
            int index = ch - 'a';
            if (node.children[index] == null) { // There is no quote on the corresponding character , Is the current node It's empty 
                return null;
            }
            node = node.children[index];
        }
        return node; // Returns the last character of a word node
    }
}

/**
 * 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 resources ：

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