Implement Trie(prefix Tree)

Problem Description

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


Trie trie = new Trie();

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

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


First understand what is trie, please read Trie (Wiki).

Now define TrieNode with char, isWord, children. 1. do insert:

  • iterate through each char in word, check whether current char ch in current node's children. if not, create new TrieNode(ch), put into current node children, other wise do nothing

  • then reset current node as curr.children.get(ch).

  • after word, set current node isWord = true.

    1. do search:

  • iterate through each char in word, for each ch:

    • if ch is in current node children, continue, if not in current node's children, return false. terminate early.

    • reset current node as curr.children.get(ch), (go next level)

    • until at last char, check current node isWord is true or not, if isWord = true, return true. otherwise no word in trie.

      1. do prefix search:

  • same search steps as search word.

  • at last step, return true (meaning it has prefix, do need to check isWord).

For example:

Complexity Analysis

insert(word): O(n) -- n is the length of word

search(word): O(n) -- n is the length of word


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 curr = root;
       for (char ch : word.toCharArray()) {
            if (!curr.children.containsKey(ch)) {
                 curr.children.put(ch, new TrieNode(ch));

            curr = curr.children.get(ch);

       curr.isWord = true;

   /** Returns if the word is in the trie. */
   public boolean search(String word) {
       return isWordOrPrefix(word, true);

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

   private boolean isWordOrPrefix(String prefix, boolean isWord) {
       TrieNode curr = root;
       for (char ch : prefix.toCharArray()) {
           if (!curr.children.containsKey(ch)) return false;
               curr = curr.children.get(ch);
       return isWord ? curr.isWord : true; 


class TrieNode {
    char ch;
    boolean isWord;
    Map<Character, TrieNode> children;
    public TrieNode(char ch) { = ch;
        isWord = false;
        children = new HashMap<>();
    public TrieNode() {
       isWord = false;
       children = new HashMap<>();

 * Your Trie object will be instantiated and called as such:
 * Trie obj = new Trie();
 * obj.insert(word);
 * boolean param_2 =;
 * boolean param_3 = obj.startsWith(prefix);

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