Implement Trie

/**
 * Your Trie object will be instantiated and called as such:
 * Trie trie = new Trie();
 * trie.insert("lintcode");
 * trie.search("lint"); will return false
 * trie.startsWith("lint"); will return true
 */
class TrieNode {
    // Initialize your data structure here.
        public char c;
        public TrieNode[] next = new TrieNode[26];
        public boolean isLeaf;
        public TrieNode() {
            
        }
        public TrieNode(char c) {
            this.c = c;
        }
        public void setToLeaf() {
            this.isLeaf = true;
        }
        public boolean isLeaf() {
            return this.isLeaf;
        }
}

public class Solution {
    private TrieNode root;

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

    // Inserts a word into the trie.
    public void insert(String word) {
        if (word == null || word.length() == 0) {
            return;
        }
        //the runner to traverse the trie
        TrieNode cur = root;
        
        //traverse the trie, if null make a new node
        for (int i = 0; i < word.length(); i++) {
            char c = word.charAt(i);
            int index = c - 'a';
            if (cur.next[index] == null) {
                cur.next[index] = new TrieNode(c);
            }
            cur = cur.next[index];
        }
        cur.setToLeaf();
    }

    // Returns if the word is in the trie.
    public boolean search(String word) {
        if (word == null || word.length() == 0) {
            return false;
        }
        //search starts from root
        TrieNode cur = root;
        
        //if this char's corresponding node is null, return false
        for (int i = 0; i < word.length(); i++) {
            char c = word.charAt(i);
            int index = c - 'a';
            if (cur.next[index] == null) {
                return false;
            }
            cur = cur.next[index];
        }
        return cur.isLeaf();
    }

    // Returns if there is any word in the trie
    // that starts with the given prefix.
    public boolean startsWith(String prefix) {
        if (prefix == null) {
            return false;
        }
        TrieNode cur = root;
        for (int i = 0; i < prefix.length(); i++) {
            char c = prefix.charAt(i);
            int index = c - 'a';
            if (cur.next[index] == null) {
                return false;
            }
            cur = cur.next[index];
        }
        return true;
    }
}