Trie树模板

本质上是个多叉树

单个节点

class TrieNode {
    public boolean isEnd;
    public TrieNode[] next;

    public TrieNode() {
        isEnd = false;
        next = new TrieNode[26];
    }
}

isEnd记录当前字符是否为某个字符串的最后一个字符
当遍历到某个isEnd==true的节点时 表明该路径到当前节点位置为止的字符依次排列 所得字串是记录过的

next数组记录当前字符是否存在 且用于存放孩子节点
记alpha为当前字符 令i = alpha - 'a' 若next[i] == null 表明“在已经遍历了某个前缀的前提下 没有以alpha字符继续的字串”

插入操作
遍历字符串每个字符
Trie树按照当前字符切换到对应的next[i]节点
若next[i]==null则新建一个子节点 并继续下渗
直到记录完所有字符 把最后一个节点的isEnd记录为true

public void insert(String word) {
    TrieNode cur = root;
    for (int i = 0; i < word.length(); i++) {
        int idx = word.charAt(i) - 'a';
        if (cur.next[idx] == null) cur.next[idx] = new TrieNode();
        cur = cur.next[idx];
    }
    cur.isEnd = true;
}

查询操作
同样都是从根节点开始 按字符下渗
如果遇到“字符串还没遍历完而没有对应的子节点”时 未记录过该字串
如果“字符串以正常遍历完” 还要判断isEnd是否为true 因为可能记录过某个以当前字串为前缀的字符串

public boolean search(String word) {
    TrieNode cur = root;
    for (int i = 0; i < word.length(); i++) {
        int idx = word.charAt(i) - 'a';
        if (cur.next[idx] == null) return false;
        cur = cur.next[idx];
    }
    return cur.isEnd;
}

查询是否有以某前缀开始的字串
查询前缀类似查询字串 但不需要判断最终是否满足isEnd == true

public boolean startsWith(String prefix) {
    TrieNode cur = root;
    for (int i = 0; i < prefix.length(); i++) {
        int idx = prefix.charAt(i) - 'a';
        if (cur.next[idx] == null) return false;
        cur = cur.next[idx];
    }
    return true;
}

完整代码

class Trie {
    class TrieNode {
        public boolean isEnd;
        public TrieNode[] next;

        public TrieNode() {
            isEnd = false;
            next = new TrieNode[26];
        }
    }

    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 cur = root;
        for (int i = 0; i < word.length(); i++) {
            int idx = word.charAt(i) - 'a';
            if (cur.next[idx] == null) cur.next[idx] = new TrieNode();
            cur = cur.next[idx];
        }
        cur.isEnd = true;
    }

    /** Returns if the word is in the trie. */
    public boolean search(String word) {
        TrieNode cur = root;
        for (int i = 0; i < word.length(); i++) {
            int idx = word.charAt(i) - 'a';
            if (cur.next[idx] == null) return false;
            cur = cur.next[idx];
        }
        return cur.isEnd;
    }

    /** Returns if there is any word in the trie that starts with the given prefix. */
    public boolean startsWith(String prefix) {
        TrieNode cur = root;
        for (int i = 0; i < prefix.length(); i++) {
            int idx = prefix.charAt(i) - 'a';
            if (cur.next[idx] == null) return false;
            cur = cur.next[idx];
        }
        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);
 */

发现查询子节点的操作很频繁 代码可以复用
修改完

class Trie {
    class TrieNode {
        public boolean isEnd;
        public TrieNode[] next;

        public TrieNode() {
            isEnd = false;
            next = new TrieNode[26];
        }
    }

    private TrieNode root;

    private TrieNode find(String str, boolean isInsert) {
        TrieNode cur = root;
        for (int i = 0; i < str.length(); i++) {
            int index = str.charAt(i) - 'a';
            if (cur.next[index] == null) {
                if (isInsert) cur.next[index] = new TrieNode();
                else return null;
            }
            cur = cur.next[index];
        }
        return cur;
    }

    /** Initialize your data structure here. */
    public Trie() {
        root = new TrieNode();
    }

    /** Inserts a word into the trie. */
    public void insert(String word) {
        find(word, true).isEnd = true;
    } 

    /** Returns if the word is in the trie. */
    public boolean search(String word) {
        TrieNode node = find(word, false);
        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) {
        return find(prefix, false) != null;
    }
}

/**
 * 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);
 */