208 Implement Trie (Prefix Tree)

208. Implement Trie (Prefix Tree)

1. Question

Implement a trie withinsert,search, andstartsWithmethods.

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

2. Implementation

class Trie {
    class TrieNode {
        char val;
        boolean isWord;
        TrieNode[] childNode;

        public TrieNode(char val) {
            this.val = val;
            childNode = new TrieNode[26];
        }
    }

    TrieNode root;

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

    /** Inserts a word into the trie. */
    public void insert(String word) {
        if (word == null || word.length() == 0) {
            return;
        }

        TrieNode curNode = root;
        for (char c : word.toCharArray()) {
            int index = c - 'a';

            if (curNode.childNode[index] == null) {
                curNode.childNode[index] = new TrieNode(c);
            }
            curNode = curNode.childNode[index];
        }
        curNode.isWord = true;
    }

    /** Returns if the word is in the trie. */
    public boolean search(String word) {
        if (word == null || word.length() == 0) {
            return false;
        }

        TrieNode curNode = root;

        for (char c : word.toCharArray()) {
            int index = c - 'a';

            if (curNode.childNode[index] == null) {
                return false;
            }
            curNode = curNode.childNode[index];
        }
        return curNode.isWord;
    }

    /** Returns if there is any word in the trie that starts with the given prefix. */
    public boolean startsWith(String prefix) {
        if (prefix == null || prefix.length() == 0) {
            return false;
        }

        TrieNode curNode = root;

        for (char c : prefix.toCharArray()) {
            int index = c - 'a';

            if (curNode.childNode[index] == null) {
                return false;
            }
            curNode = curNode.childNode[index];
        }
        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);
 */

3. Time & Space Complexity

时间复杂度: inset:O(L), search: O(L), startsWith: O(L), L 为输入string的长度

空间复杂度: O(L)