Question:

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

Note:
You may assume that all inputs are consist of lowercase letters a-z.

Thinking:

I assume every node has no val, but edges do. So I keep a HashMap for everynode. What’s more, in order to determine whether this node is a leave, I add another boolean value.

Solution:

import java.util.HashMap;

class TrieNode {
    HashMap<Character, TrieNode> edges = new HashMap<Character, TrieNode>();
    boolean leave = false;
    // Initialize your data structure here.
    public TrieNode() {
    }
}

public class Trie {
    private TrieNode root;

    public Trie() {
        root = new TrieNode();
    }

    // Inserts a word into the trie.
    public void insert(String word) {
        TrieNode cur = root;
        for (char c: word.toCharArray()){
            if (!cur.edges.containsKey(c)){
                TrieNode newNode = new TrieNode();
                cur.edges.put(c, newNode);
                cur = newNode;

            }
            else {
                cur = cur.edges.get(c);
            }
        }
        cur.leave = true;
    }

    // Returns if the word is in the trie.
    public boolean search(String word) {
        TrieNode cur = root;
        for (char c: word.toCharArray()){
            if (!cur.edges.containsKey(c)){
                return false;
            }
            else{
                cur = cur.edges.get(c);
            }
        }
        return cur.leave;
    }

    // Returns if there is any word in the trie
    // that starts with the given prefix.
    public boolean startsWith(String prefix) {
        TrieNode cur = root;
        for (char c: prefix.toCharArray()){
            if (!cur.edges.containsKey(c)){
                return false;
            }
            else{
                cur = cur.edges.get(c);
            }
        }
        return true;
    }
}

// Your Trie object will be instantiated and called as such:
// Trie trie = new Trie();
// trie.insert("somestring");
// trie.search("key");