Trie 树

Leetcode: https://leetcode.com/problems/implement-trie-prefix-tree/

class Trie {
    Character curChar;
    boolean isEnd;
    Map<Character, Trie> children = new HashMap<>();

    public Trie() {}
    
    public Trie(char ch){
        curChar = ch;
        isEnd = false;
    }
    
    public void insert(String word) {
        if(word == null || word.isEmpty()){
            //here to mark the end of a word for the last char
            isEnd = true;
            return;
        }
        char cur = word.charAt(0);
        if(children.containsKey(cur)){
            Trie child = children.get(cur);
            child.insert(word.substring(1));
        }else{
            Trie newNode = new Trie(cur);
            children.put(cur,newNode);
            newNode.insert(word.substring(1));
        }

    }
    
    public boolean search(String word) {
        return searchHelper(word, false);
    }
    
    public boolean startsWith(String prefix) {
        return searchHelper(prefix, true);
    }
    
    private boolean searchHelper(String word, boolean isPrefixSearch){
        if(word == null || word.isEmpty()){
            return false;
        }
        char cur = word.charAt(0);
        if(children.containsKey(cur)){
            if(word.length() == 1){
                //for the last char of the word
                Trie child = children.get(cur);
                boolean end = child.children.isEmpty()|child.isEnd;
                return isPrefixSearch?true:end;
            }else{
                return children.get(cur).searchHelper(word.substring(1), isPrefixSearch);
            }
        }else{
            return false;
        }
        
    }
    
}



/**
 * Your Trie object will be instantiated and called as such:
 * Trie obj = new Trie();
 * obj.insert(word);
 * boolean param_2 = obj.search(word);
 * boolean param_3 = obj.startsWith(prefix);
 */

 

 

 

 

实现一个trie树，快速返回包含某个前缀的单词数量。

http://hihocoder.com/problemset/problem/1014

 

import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintStream;
import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) throws FileNotFoundException {
        TrieNode root = new TrieNode('1');

        // Scanner in = new Scanner(new File("A.in"));
        // PrintStream out = new PrintStream(new File("A.out"));
        Scanner in = new Scanner(System.in);
        PrintStream out = new PrintStream(System.out);

        int n = in.nextInt();
        for (int i = 0; i < n; i++) {
            String str = in.next();
            root.insert(str);
        }

        int m = in.nextInt();
        for (int i = 0; i < m; i++) {
            String str = in.next();
            out.println(root.getCount(str));
        }

    }
}

class TrieNode {
    Character curChar;
    Map<Character, TrieNode> children = new HashMap<Character, TrieNode>();
    int count = 1;

    public TrieNode(char ch) {
        curChar = ch;
    }

    public void insert(String str) {
        if (str.isEmpty())
            return;
        char cur = str.charAt(0);
        if (children.containsKey(cur)) {
            TrieNode child = children.get(cur);
            child.count++;
            child.insert(str.substring(1));
        } else {
            TrieNode newNode = new TrieNode(cur);
            children.put(cur, newNode);
            newNode.insert(str.substring(1));
        }
    }

    public int getCount(String str) {
        if (str == null)
            return 0;
        if (str.isEmpty()) {
            return count;
        }
        char ch = str.charAt(0);
        if (!children.containsKey(ch)) {
            return 0;
        } else
            return children.get(ch).getCount(str.substring(1));
    }
}