Leetcode 208: Implement Trie (Prefix Tree)

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

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

 

public class Trie {
 // prefix tree node
    public class TreeNode
    {
        public bool isWord;
        public TreeNode[] children;
        public char val;
        
        public TreeNode(char val, bool isWord)
        {
            this.val = val;
            this.isWord = isWord;
            this.children = new TreeNode[26];
        }
        
        public void AddChild(char c)
        {
            int index = this.GetIndex(c);
            
            if (this.children[index] == null)
            {
                this.children[index] = new TreeNode(c, false);
            }
        }
        
        public TreeNode GetChild(char c)
        {
            return this.children[this.GetIndex(c)];
        }
        
        private int GetIndex(char c)
        {
            return (int)c - (int)'a';
        }
    }
    
    private TreeNode root = new TreeNode('*', false);
    
    /** Initialize your data structure here. */
    public Trie() {
        
    }
    
    /** Inserts a word into the trie. */
    public void Insert(string word) {
        this.InsertWord(this.root, word, 0);
    }
    
    private void InsertWord(TreeNode node, string s, int start)
    {
        if (start >= s.Length)
        {
            node.isWord = true;
            return;
        }
        
        char c = s[start];
        
        node.AddChild(c);
        
        InsertWord(node.GetChild(c), s, start + 1);
    }
    
    /** Returns if the word is in the trie. */
    public bool Search(string word) {
        return this.DFS(this.root, word, 0, false);
    }
    
    private bool DFS(TreeNode node, string s, int start, bool prefix)
    {
        if (start >= s.Length)
        {
            return prefix || node.isWord;
        }
        
        char c = s[start];
        
        if (node.GetChild(c) == null) return false;
        
        return DFS(node.GetChild(c), s, start + 1, prefix);
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    public bool StartsWith(string prefix) {
        return this.DFS(this.root, prefix, 0, true);
    }
}


/**
 * Your Trie object will be instantiated and called as such:
 * Trie obj = new Trie();
 * obj.Insert(word);
 * bool param_2 = obj.Search(word);
 * bool param_3 = obj.StartsWith(prefix);
 */