LeetCode 208. Implement Trie (Prefix Tree)
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Implement a trie with insert
, search
, and startsWith
methods.
Note:
You may assume that all inputs are consist of lowercase letters a-z
.
这道题让我们实现一个重要但又有些复杂的数据结构-字典树, 又称前缀树或单词查找树,详细介绍可以参见网友董的博客,例如,一个保存了8个键的trie结构,"A", "to", "tea", "ted", "ten", "i", "in", and "inn".如下图所示:
字典树主要有如下三点性质:
1. 根节点不包含字符,除根节点意外每个节点只包含一个字符。
2. 从根节点到某一个节点,路径上经过的字符连接起来,为该节点对应的字符串。
3. 每个节点的所有子节点包含的字符串不相同。
字母树的插入(Insert)、删除( Delete)和查找(Find)都非常简单,用一个一重循环即可,即第i 次循环找到前i 个字母所对应的子树,然后进行相应的操作。实现这棵字母树,我们用最常见的数组保存(静态开辟内存)即可,当然也可以开动态的指针类型(动态开辟内存)。至于结点对儿子的指向,一般有三种方法:
1、对每个结点开一个字母集大小的数组,对应的下标是儿子所表示的字母,内容则是这个儿子对应在大数组上的位置,即标号;
2、对每个结点挂一个链表,按一定顺序记录每个儿子是谁;
3、使用左儿子右兄弟表示法记录这棵树。
两种方法,各有特点。第一种易实现,但实际的空间要求较大;第二种,空间要求最小,但相对费时且不易写。
1.我们先来看第一种实现方法,这种方法实现起来简单直观,字母的字典树每个节点要定义一个大小为26的子节点指针数组,然后用一个标志符用来记录到当前位置为止是否为一个词,初始化的时候讲26个子节点都赋为空。那么insert操作只需要对于要插入的字符串的每一个字符算出其的位置,然后找是否存在这个子节点,若不存在则新建一个,然后再查找下一个。查找词和找前缀操作跟insert操作都很类似,不同点在于若不存在子节点,则返回false。查找次最后还要看标识位,而找前缀直接返回true即可
1 class TrieNode { 2 public char val; 3 public boolean isWord; 4 public TrieNode[] children = new TrieNode[26]; 5 public TrieNode() {} 6 TrieNode(char c){ 7 TrieNode node = new TrieNode(); 8 node.val = c; 9 } 10 } 11 12 public class Trie { 13 private TrieNode root; 14 public Trie() { 15 root = new TrieNode(); 16 root.val = ‘ ‘; 17 } 18 19 public void insert(String word) { 20 TrieNode ws = root; 21 for(int i = 0; i < word.length(); i++){ 22 char c = word.charAt(i); 23 if(ws.children[c - ‘a‘] == null){ 24 ws.children[c - ‘a‘] = new TrieNode(c); 25 } 26 ws = ws.children[c - ‘a‘]; 27 } 28 ws.isWord = true; 29 } 30 31 public boolean search(String word) { 32 TrieNode ws = root; 33 for(int i = 0; i < word.length(); i++){ 34 char c = word.charAt(i); 35 if(ws.children[c - ‘a‘] == null) return false; 36 ws = ws.children[c - ‘a‘]; 37 } 38 return ws.isWord; 39 } 40 41 public boolean startsWith(String prefix) { 42 TrieNode ws = root; 43 for(int i = 0; i < prefix.length(); i++){ 44 char c = prefix.charAt(i); 45 if(ws.children[c - ‘a‘] == null) return false; 46 ws = ws.children[c - ‘a‘]; 47 } 48 return true; 49 } 50 }
2.
1 class Trie { 2 private String letter; 3 private List<Trie> children; 4 private boolean end; 5 /** Initialize your data structure here. */ 6 public Trie() { 7 this.letter = ""; 8 this.children = new ArrayList<Trie>(); 9 boolean end = false; 10 } 11 12 public Trie(String letter) { 13 this.letter = letter; 14 this.children = new ArrayList<Trie>(); 15 this.end = false; 16 } 17 18 /** Inserts a word into the trie. */ 19 public void insert(String word) { 20 insertHelper(this, word, 0); 21 } 22 23 /** Returns if the word is in the trie. */ 24 public boolean search(String word) { 25 boolean result = true; 26 result = result && searchHelper(this, word, 0); 27 return result; 28 } 29 30 /** Returns if there is any word in the trie that starts with the given prefix. */ 31 public boolean startsWith(String prefix) { 32 if (prefix.length() == 0) 33 return true; 34 boolean result = false; 35 for(Trie trie: children){ 36 if (trie.letter.equals(prefix.charAt(0) + "")){ 37 result = true && startsWithHelper(trie, prefix, 1); 38 if(result == false) 39 return false; 40 } 41 } 42 return result; 43 } 44 45 private boolean startsWithHelper(Trie trie, String prefix, int pos){ 46 if(pos == prefix.length()) 47 return true; 48 boolean result = false; 49 for(Trie child: trie.children){ 50 if(child.letter.equals(prefix.charAt(pos) + "")){ 51 result = true && startsWithHelper(child, prefix, pos +1); 52 if(result == false) 53 return false; 54 } 55 } 56 return result; 57 } 58 59 private void insertHelper(Trie trie, String word, int pos){ 60 boolean has = false; 61 if(pos == word.length()) 62 return; 63 int len = trie.children.size(); 64 for(int i=0; i < len; i++){ 65 Trie child = trie.children.get(i); 66 if(child.letter.equals(word.charAt(pos) + "")){ 67 has = true; 68 if(pos == word.length()-1){ 69 if(child.end == false){ 70 child.end = true; 71 } 72 } 73 insertHelper(child, word, pos+1); 74 } 75 } 76 if(!has) 77 create(trie,word,pos); 78 } 79 80 private void create(Trie parent, String word, int pos){ 81 if(pos == word.length()) 82 return; 83 Trie child = new Trie(word.charAt(pos) + ""); 84 if(pos == word.length() -1) 85 child.end = true; 86 parent.children.add(child); 87 create(child, word, pos+1); 88 } 89 90 private boolean searchHelper(Trie trie, String word, int pos){ 91 if(pos == word.length() && trie.children.size() == 0) 92 return true; 93 else if(pos == word.length()) 94 return false; 95 boolean result = false; 96 for(Trie child: trie.children){ 97 if(child.letter.equals(word.charAt(pos) + "")){ 98 if(pos == word.length() -1){ 99 if(child.end == true) 100 return true; 101 else 102 return false; 103 } 104 result = true && searchHelper(child, word, pos +1); 105 if(result == false) 106 return false; 107 } 109 } 110 return result; 111 } 112 }
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