Leetcode 208 solution: Implement Trie (Prefix Tree)

阅读原文

https://blog.baozitraining.org/2019/04/leetcode-solution-208-implement-trie.html

Problem Statement

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.

Problem link

Video Tutorial

You can find the detailed video tutorial here

Thought Process

Trie (prefix tree) is a very common data structure that appears very often in interviews. I used to get asked Trie at Fitbit, Snap and Google back in the days. I highly recommend you brush up on this basic data structure. Please refer to this Baozi Training Blog for a modified version explained: https://blog.baozitraining.org/2019/03/leetcode-solution-211-add-and-search.html Leetcode actually did a great job explaining this problem. Feel free to download the explanation here (all credit and copyright goes to leetcode, obviously)

public class TrieNode {
   public Character letter = null;
       // a boolean to indicate if this node is end of a word, sometimes we want to match word exactly, not just prefix
   public boolean isEnd = false;
   
   public Map<Character, TrieNode> children = null;
   // Initialize your data structure here.
   public TrieNode() {
      this.children = new HashMap<Character, TrieNode>();
   }
}


public TrieNode root;
public ImplementTrie() {
       // This root node essentially is a dummy root
   this.root = new TrieNode();
}

   // This is my fitbit onsite version, looks cool
   public void insert(String word) {
       if (word == null || word.length() == 0) {
           return;
       }

       TrieNode cur = this.root;
       int i = 0;
       while (i < word.length()) {
           char c = word.charAt(i);
           if (!cur.children.containsKey(c)) {
               cur.children.put(c, new TrieNode());
           }
           cur = cur.children.get(c);
           i++;
       }
       cur.isEnd = true;
   }


   // Returns if the word is in the trie, it has to be the exact word
   public boolean search(String word) {
       if (word == null || word.length() == 0) {
           return false;
       }

       int i = 0;
       TrieNode cur = this.root;
       while (i < word.length()) {
           char c = word.charAt(i);
           if (!cur.children.containsKey(c)) {
               return false;
           }

           cur = cur.children.get(c);
           i++;
       }

       return cur.isEnd;
   }

   // 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 i = 0;
       TrieNode cur = this.root;
       // Nice, this means if prefix is empty, it can match everything, if it's " " space, then it won't match unless there is a space
       while (i < prefix.length()) {
           char c = prefix.charAt(i);
           if (!cur.children.containsKey(c)) {
               return false;
           }

           cur = cur.children.get(c);
           i++;
       }

       return true;
   }

Time Complexity: assuming N is the word length, insert O(N), search O(N), startWith O(N) Space Complexity: assuming N is the word length, insert O(N), search O(1), startWith O(1)

References

Leetcode official solution