Trie Implementation

Trie.java

package build;
import java.util.*;
public class Trie {
	private int SIZE = 26;
	private TrieNode root;
	Trie()
	{
		root = new TrieNode();
	}
	private class TrieNode{
		private int count;
		private TrieNode[] children;
		private boolean isLeaf;
		private char ch;
		TrieNode()
		{
			count=1;
			children = new TrieNode[SIZE];
			isLeaf = false;
		}
	}
	
	public void insert(String s)
	{
		if(s==null || s.length()<=0) 
			return;
		TrieNode node = root;
		char[] letters = s.toCharArray();
		for(int i=0;i<s.length();++i)
		{
			int pos = letters[i]-'a';
			if(node.children[pos]==null)
			{
				node.children[pos] = new TrieNode();
				node.children[pos].ch = letters[i];
			}
			else
			{
				node.children[pos].count++;
			}
			node = node.children[pos];
				
		}
		node.isLeaf=true;
	}
	
	public int countPrefix(String prefix)
	{
		if(prefix==null || prefix.length()<=0)
			return -1;
		TrieNode node = root;
		char[] letters = prefix.toCharArray();
		for(int i=0;i<prefix.length();++i)
		{
			int pos = letters[i]-'a';
			if(node.children[pos]==null)
				return 0;
			else
				node = node.children[pos];
		}
		return node.count;
	}
	
	public boolean has(String s)
	{
		if(s==null || s.length()<=0) 
			return false;
		TrieNode node = root;  
        char[] letters=s.toCharArray();  
        for (int i = 0, len = s.length(); i < len; i++) {  
            int pos = letters[i] - 'a';  
            if (node.children[pos] != null) {  
                node = node.children[pos];  
            } else {  
                return false;  
            }  
        }  
        return node.isLeaf;  
	}
	
	public void preTraverse(TrieNode node)
	{
		if(node!=null)
		{
			System.out.println(node.ch+' ');
			for(TrieNode child:node.children)
			{
				preTraverse(child);
			}
		}
	}
}