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  • [LeetCode] 208. Implement Trie (Prefix Tree) ☆☆☆

    Implement a trie with insertsearch, and startsWith methods.

    Note:
    You may assume that all inputs are consist of lowercase letters a-z.

    解法:

      Trie(字典树)的知识参见:数据结构之Trie树 和 [LeetCode] Implement Trie (Prefix Tree) 实现字典树(前缀树)

      可以采用数组和哈希表的方式实现,代码分别如下:

    public class Trie {
        private TrieNode root;
    
        /** Initialize your data structure here. */
        public Trie() {
            root = new TrieNode();
        }
        
        /** Inserts a word into the trie. */
        public void insert(String word) {
            root.insert(word, 0);
        }
        
        /** Returns if the word is in the trie. */
        public boolean search(String word) {
            TrieNode result = root.search(word, 0);
            return result != null && result.getIsWord();
        }
        
        /** Returns if there is any word in the trie that starts with the given prefix. */
        public boolean startsWith(String prefix) {
            TrieNode result = root.search(prefix, 0);
            return result != null;
        }
    }
    
    class TrieNode {
        private TrieNode[] children;
        private boolean isWord;
        
        public TrieNode() {
            children = new TrieNode[26];
            isWord = false;
        }
        
        public void insert(String word, int index) {
            // 如果所有字符都已插入,需要将最后一个字符节点的isWord改为true
            if (index == word.length()) {
                this.isWord = true;
                return;
            }
            // 如果不存在该字符,在对应位置新建节点
            int n = word.charAt(index) - 'a';
            if (children[n] == null) {
                children[n] = new TrieNode();
            }
            // 继续下一字符
            children[n].insert(word, index + 1);
        }
        
        // 由于Trie中既要求实现search,又要求实现startsWith,为了方便,
        // 此处直接返回搜索结果的TrieNode,交由Trie去判断。
        public TrieNode search(String word, int index) {
            if (index == word.length()) {
                return this;
            }
            int n = word.charAt(index) - 'a';
            if (children[n] == null) {
                return null;
            }
            return children[n].search(word, index + 1);
        }
        
        public boolean getIsWord() {
            return this.isWord;
        }
    }
    
    /**
     * 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);
     */
    public class Trie {
        private TrieNode root;
    
        /** Initialize your data structure here. */
        public Trie() {
            root = new TrieNode();
        }
        
        /** Inserts a word into the trie. */
        public void insert(String word) {
            TrieNode curr = root;
            for (int i = 0; i < word.length(); i++) {
                char letter = word.charAt(i);
                if (!curr.children.containsKey(letter)) {
                    curr.children.put(letter, new TrieNode());
                }
                curr = curr.children.get(letter);
            }
            curr.isWord = true;
        }
        
        /** Returns if the word is in the trie. */
        public boolean search(String word) {
            TrieNode result = find(word);
            return result != null && result.isWord;
        }
        
        /** Returns if there is any word in the trie that starts with the given prefix. */
        public boolean startsWith(String prefix) {
            TrieNode result = find(prefix);
            return result != null;
        }
        
        public TrieNode find(String word) {
            TrieNode curr = root;
            for (int i = 0; i < word.length(); i++) {
                char letter = word.charAt(i);
                if (!curr.children.containsKey(letter)) {
                    return null;
                }
                curr = curr.children.get(letter);
            }
            return curr;
        }
    }
    
    class TrieNode {
        HashMap<Character, TrieNode> children;
        boolean isWord;
        
        public TrieNode() {
            children = new HashMap<>();
            isWord = 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);
     */
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  • 原文地址:https://www.cnblogs.com/strugglion/p/6426979.html
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