題目

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

題解

第一版：暴力法

import java.util.LinkedHashSet;class Trie {LinkedHashSet<String> set;/** Initialize your data structure here. */public Trie() {set = new LinkedHashSet<>();}/** Inserts a word into the trie. */public void insert(String word) {set.add(word);}/** Returns if the word is in the trie. */public boolean search(String word) {return set.contains(word);}/** Returns if there is any word in the trie that starts with the given prefix. */public boolean startsWith(String prefix) {for (String s : set)if (s.startsWith(prefix)) return true;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);*/

第二版：前綴樹的實現

參考：官方題解

Trie node structure

Insertion of a key to a trie

Search for a key in a trie

Search for a key prefix in a trie

My Code

class Node{Node[] map;boolean isEnd;public Node() {this.map = new Node['z' - 'a' + 1];}public Node get(char c) {return map[c - 'a'];}public Node put(char c) {Node node = new Node();map[c - 'a'] = node;return node;} } class Trie {Node node;/** Initialize your data structure here. */public Trie() {node=new Node();}/** Inserts a word into the trie. */public void insert(String word) {char[] chars = word.toCharArray();Node cur = node;for (int i = 0; i < chars.length; i++) {if (cur.get(chars[i]) != null)cur = cur.get(chars[i]);elsecur = cur.put(chars[i]);if (i == chars.length - 1)cur.isEnd = true;}}/** Returns if the word is in the trie. */public boolean search(String word) {Node node = searchPrefix(word);return node != null && node.isEnd;}/** Returns if there is any word in the trie that starts with the given prefix. */public boolean startsWith(String prefix) {Node node = searchPrefix(prefix);return node != null;}public Node searchPrefix(String prefix) {char[] chars = prefix.toCharArray();Node cur = node;for (char c : chars) {if (cur.get(c) == null) return null;// cur.isEnd可能是偽結束符，不能用來判斷是否真正結束遍歷else cur = cur.get(c);}return cur;} }/*** 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);*/

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附：左神算法 - 子數組最大異或和

package chapter_5_stringproblem;public class Problem_24_TrieTree {public static class TrieNode {public int path;public int end;public TrieNode[] map;public TrieNode() {path = 0;end = 0;map = new TrieNode[26];}}public static class Trie {private TrieNode root;public Trie() {root = new TrieNode();}public void insert(String word) {if (word == null) {return;}char[] chs = word.toCharArray();TrieNode node = root;node.path++;int index = 0;for (int i = 0; i < chs.length; i++) {index = chs[i] - 'a';if (node.map[index] == null) {node.map[index] = new TrieNode();}node = node.map[index];node.path++;}node.end++;}public void delete(String word) {if (search(word)) {char[] chs = word.toCharArray();TrieNode node = root;node.path--;int index = 0;for (int i = 0; i < chs.length; i++) {index = chs[i] - 'a';if (node.map[index].path-- == 1) {node.map[index] = null;return;}node = node.map[index];}node.end--;}}public boolean search(String word) {if (word == null) {return false;}char[] chs = word.toCharArray();TrieNode node = root;int index = 0;for (int i = 0; i < chs.length; i++) {index = chs[i] - 'a';if (node.map[index] == null) {return false;}node = node.map[index];}return node.end != 0;}public int prefixNumber(String pre) {if (pre == null) {return 0;}char[] chs = pre.toCharArray();TrieNode node = root;int index = 0;for (int i = 0; i < chs.length; i++) {index = chs[i] - 'a';if (node.map[index] == null) {return 0;}node = node.map[index];}return node.path;}}public static void main(String[] args) {Trie trie = new Trie();System.out.println(trie.search("zuo"));trie.insert("zuo");System.out.println(trie.search("zuo"));trie.delete("zuo");System.out.println(trie.search("zuo"));trie.insert("zuo");trie.insert("zuo");trie.delete("zuo");System.out.println(trie.search("zuo"));trie.delete("zuo");System.out.println(trie.search("zuo"));trie.insert("zuoa");trie.insert("zuoac");trie.insert("zuoab");trie.insert("zuoad");trie.delete("zuoa");System.out.println(trie.search("zuoa"));System.out.println(trie.prefixNumber("zuo"));}}

總結

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