208. Implement Trie (Prefix Tree)

Posted 2020-09-29 白天黑夜每日c

Implement a trie with insert, search, and startsWith methods.

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

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这棵Ｔrie树里存的都是英文小写字母，因此每个Node有２６个孩子，代表它的下一个字符a-z。同时，每个节点中，用ｃｏｕｎｔ记录到这个节点是否是一个endpoint

** 这里没有对Ｔｒｉｅ树做删除的操作。在查找prefix时，我是判断如果对prefix里的字符一个个遍历，如果节点都不为空，表示存在以Prefix为前缀的字符串。*如果加上删除操作，要确保把Ｔｒｉｅ里的整条路径删除。

思考：Ｔrie Tree的优缺点有什么?

public class Trie {
    Trie[] children;
    int count = 0;
    /** Initialize your data structure here. */
    public Trie() {
        children = new Trie[26];
    }
    
    /** Inserts a word into the trie. */
    public void insert(String word) {
        if (word.equals("")) {
            count++;
        }
        Trie cur = this;
        int i = 0;
        while (i < word.length()) {
            if (cur.children[word.charAt(i) - ‘a‘] == null) {
                cur.children[word.charAt(i) - ‘a‘] = new Trie();
            }
            cur = cur.children[word.charAt(i) - ‘a‘];
            i++;
        }
        cur.count++;
    }
    
    /** Returns if the word is in the trie. */
    public boolean search(String word) {
        Trie cur = this;
        int i = 0;
        while (cur != null && i < word.length()) {
            cur = cur.children[word.charAt(i) - ‘a‘];
            i++;
        }
        if (cur == null || cur.count == 0) {
            return false;
        }
        return true;
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    public boolean startsWith(String prefix) {
        Trie cur = this;
        int i = 0;
        while (cur != null && i < prefix.length()) {
            cur = cur.children[prefix.charAt(i) - ‘a‘];
            i++;
        }
        if (cur == null) {
            return false;
        }
        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);
 */