字典树trie

字典树经常用于单词搜索，现在网络引擎中也应用了trie树；

public class Trie{
    private int SIZE = 26;
    private TrieNode root;
    Trie(){
        root = new TrieNode();
    }
    private class TrieNode{
        private int num;//the times  that words passing this node
        private TrieNode[] son;//son point
        private boolean isEnd;//record weather there are points ending in this point
        private char val;//char value
        public TrieNode(){
            num = 1;
            son = new TrieNode[SIZE];
            isEnd = false;
        }
    }
    //create the trie tree
    public void insert(String str){//insert a word to the trie tree
        if(str == null||str.length() == 0){
            return;
        }
        TrieNode node = root;
        char[] letters = str.toCharArray();
        for(int i =0,len = str.length();i<len;i++){
            int pos = letters[i]-'a';
            if(node.son[pos]==null){
                node.son[pos] = new TrieNode();
                node.son[pos].val = letters[i];
            }else{
                node.son[pos].num++;
            }
            node = node.son[pos];
        }
        node.isEnd= true;//
    }
    //calculate the count of words'prefix
    public int countPrefix(String prefix){
        if(prefix == null||prefix.length()==0){
            return -1;
        }
        TrieNode node = root;
        char[] letters = prefix.toCharArray();
        int pos;
        int num = 0;
        for(int i=0,len = prefix.length();i<len;i++){
            pos = letters[i]-'a';
            node = node.son[pos];
            if(node==null){
                return 0;
            }else{
                num = node.num;
            }
        }
        return num;
    }
    //print the word having the assigned prefix
    public String hasprefix(String prefix){
        if(prefix ==null||prefix.length()==0){
            return null;
        }
        TrieNode node = root;
        char[] letters = prefix.toCharArray();
        int pos;
        for(int i=0,len = prefix.length();i<len; i++){
            pos = letters[i]-'a';
            if(node.son[pos]==null){
                return null;
            }else{
                node = node.son[pos];
            }
        }
        preTraverse(node,prefix);
        return null;
    }
    //walk the node's words
    public void preTraverse(TrieNode node,String prefix){
        if(!node.isEnd){
            for(TrieNode child:node.son){
                if(child!=null){
                    preTraverse(child,prefix+child.val);
                }
            }
            return;
        }
        System.out.println(prefix);
    }
    //find the completely matching word
    public boolean has(String str){
        if(str == null||str.length() == 0){
            return false;
        }
        TrieNode node = root;
        char[] letters = str.toCharArray();
        int pos;
        for(int i=0,len = str.length();i<len;i++){
            pos = letters[i]-'a';
            if(node.son[pos]==null){
                return false;
            }else{
                node = node.son[pos];
            }
        }
        return node.isEnd;
    }
    //pre-order of the trie tree
    public void preTraverse(TrieNode node){
        if(node!=null){
            System.out.print(node.val+" ");
            for(TrieNode child:node.son){
                preTraverse(child);
            }
        }
    }
    public TrieNode getRoot(){
        return this.root;
    }
    public static void main(String[] args){
        Trie tree = new Trie();
        String[] strs = {"dsfahjk","fjdkafhdask","fdhjfk","dafjdk","qepoi","fda"};
        String[] prefix={"ds","f","qepo","abd"};
        for(String str:strs){
            tree.insert(str);
        }
        for(String pre:prefix){
            int num = tree.countPrefix(pre);
            System.out.println(pre+" "+num);
        }
        tree.preTraverse(tree.getRoot());
        System.out.println("\ntree.has(\"f\"):"+tree.has("f"));
        System.out.println("tree.has(\"fda\"):"+tree.has("fda"));
        tree.hasprefix("f");
        System.out.println("countprefix(\"f\"):"+tree.countPrefix("f"));
    }
}