[LeetCode 208] Implement Trie (Prefix Tree)

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

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

class TrieNode {
    boolean exist = false;
    TrieNode[] children;
    char val;
    // Initialize your data structure here.
    public TrieNode() {
        
    }
    public TrieNode(char c) {
        this.val = c;
    }
}

public class Trie {
    private TrieNode root;

    public Trie() {
        root = new TrieNode();
    }

    // Inserts a word into the trie.
    public void insert(String word) {
        if(word == null || word.length()<=0) return;
        TrieNode pre = root;
        for(int i=0;i<word.length();i++){
            if(pre.children == null){
                pre.children = new TrieNode[26];
            }
            int index = word.charAt(i)-'a';
            if(pre.children[index]==null){
                pre.children[index] = new TrieNode(word.charAt(i));
            }
            pre = pre.children[index];
            if(i == word.length()-1){
                pre.exist = true;
            }
        }
    }

    // Returns if the word is in the trie.
    public boolean search(String word) {
        if(word == null || word.length() <= 0) return false;
        TrieNode pre = root;
        for(int i=0;i<word.length();i++){
            int index = word.charAt(i)-'a';
            if(pre.children == null || pre.children[index]==null){
                return false;
            }
            if(i == word.length()-1 && pre.children[index].exist == false){
                return false;
            }
            pre = pre.children[index];
        }
        return true;
    }

    // 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 pre = root;
        for(int i=0;i<prefix.length();i++){
            int index = prefix.charAt(i)-'a';
            if(pre.children == null || pre.children[index]==null){
                return false;
            }
            pre = pre.children[index];
        }
        return true;
    }
}

// Your Trie object will be instantiated and called as such:
// Trie trie = new Trie();
// trie.insert("somestring");
// trie.search("key");

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