Trie

前缀树主要用于解决与字符串查找相关d

剑指 Offer II 062. 实现前缀树

方法一：前缀树

class Trie {
    static class TrieNode {
        private TrieNode[] children = new TrieNode[26];
        private boolean isWord;
    }
    /** Initialize your data structure here. */
    private TrieNode root = new TrieNode();
    
    /** Inserts a word into the trie. */
    public void insert(String word) {
        TrieNode node = root;
        for (char ch : word.toCharArray()) {
            if (node.children[ch - 'a'] == null)
                node.children[ch - 'a'] = new TrieNode();
            node = node.children[ch - 'a'];
        }
        node.isWord = true;
    }
    
    /** Returns if the word is in the trie. */
    public boolean search(String word) {
        TrieNode node = root;
        for (char ch : word.toCharArray()) {
            if (node.children[ch - 'a'] == null)
                return false;
            node = node.children[ch - 'a'];
        }
        return node.isWord;
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    public boolean startsWith(String prefix) {
        TrieNode node = root;
        for (char ch : prefix.toCharArray()) {
            if (node.children[ch - 'a'] == null)
                return false;
            node = node.children[ch - 'a'];
        }
        return true;
    }
}

/**
 * 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);
 */

剑指 Offer II 063. 替换单词

一刷：

class Solution {
    static class TrieNode {
        private TrieNode[] children;
        private boolean isWord;
        public TrieNode() {
            children = new TrieNode[26];
        }
    }


    public String replaceWords(List<String> dictionary, String sentence) {
        TrieNode root = buildTrie(dictionary);
        String[] words = sentence.split(" ");
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < words.length; ++i) {
            String prefix = findPrefix(root, words[i]);
            if (!prefix.isEmpty()) {
                words[i] = prefix;
            }
        }
        return String.join(" ", words);
    }

    private String findPrefix(TrieNode root, String word) {
        TrieNode cur = root;
        StringBuilder sb = new StringBuilder();
        for (char ch : word.toCharArray()) {
            if (cur.children[ch - 'a'] == null || cur.isWord)
                break;
            sb.append(ch);
            cur = cur.children[ch - 'a'];
        }
        return cur.isWord == true ? sb.toString() : "";
    }

    private TrieNode buildTrie(List<String> dictionary) {
        TrieNode root = new TrieNode();
        for (String str : dictionary) {
            TrieNode cur = root;
            for (char ch : str.toCharArray()) {
                if (cur.children[ch - 'a'] == null)
                    cur.children[ch - 'a'] = new TrieNode();
                cur = cur.children[ch - 'a'];
            }
            cur.isWord = true;
        }
        return root;
    }
}

二刷

class Solution {
    static class TrieNode {
        TrieNode[] children = new TrieNode[26];
        boolean isWord;
    }
    TrieNode root = new TrieNode();
    private void buildDict(List<String> dictionary) {
        for (String word : dictionary) {
            TrieNode node = root;
            for (char ch : word.toCharArray()) {
                if (node.children[ch - 'a'] == null)
                    node.children[ch - 'a'] = new TrieNode();
                node = node.children[ch - 'a'];
            }
            node.isWord = true;
        }
    }

    public String replaceWords(List<String> dictionary, String sentence) {
        buildDict(dictionary);
        String[] words = sentence.split(" ");
        StringBuilder res = new StringBuilder(), sb = new StringBuilder();
        for (String word : words) {
            TrieNode node = root;
            for (char ch : word.toCharArray()) {
                if (node.children[ch - 'a'] == null)
                    break;
                sb.append(ch);
                node = node.children[ch - 'a'];
                if (node.isWord) {
                    sb.append(" ");
                    res.append(sb);
                    sb = new StringBuilder();
                    break;
                }
            }
            sb = new StringBuilder();
            if (!node.isWord)
                res.append(word + " ");
        }
        res.deleteCharAt(res.length() - 1);
        return res.toString();
    }
}

剑指 Offer II 064. 神奇的字典

方法一：前缀树

注意：一定要判断root.isWord，比如字典里有“hello”，search(“hhll”)

if (root.isWord && index == searchWord.length() && diff == 1)
    return true;

---

class MagicDictionary {
    
    static class TrieNode {
        TrieNode[] children = new TrieNode[26];
        boolean isWord;
    } 
    /** Initialize your data structure here. */
    TrieNode root = new TrieNode();
    
    public void buildDict(String[] dictionary) {
        for (String word : dictionary) {
            TrieNode node = root;
            for (char ch : word.toCharArray()) {
                if (node.children[ch - 'a'] == null)
                    node.children[ch - 'a'] = new TrieNode();
                node = node.children[ch - 'a'];
            }
            node.isWord = true;
        }
    }
    
    public boolean search(String searchWord) {
        return dfs(root, searchWord, 0, 0);
    }

    private boolean dfs(TrieNode root, String searchWord, int index, int diff) {
        if (root == null || diff > 1)
            return false;
        if (root.isWord && index == searchWord.length() && diff == 1)
            return true;
        if (index == searchWord.length())
            return false;
        boolean found = false;
        for (int i = 0; i < 26; ++i) {
            if (found)
                break;
            int nextDiff = i == searchWord.charAt(index) - 'a' ? diff : diff + 1;
            found = dfs(root.children[i], searchWord, index + 1, nextDiff);
        }
        return found;
    }
}
/**
 * Your MagicDictionary object will be instantiated and called as such:
 * MagicDictionary obj = new MagicDictionary();
 * obj.buildDict(dictionary);
 * boolean param_2 = obj.search(searchWord);
 */