Implement Trie (Prefix Tree)

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

Example 1
insert("apple"), search("apple")→true, search("app")→false, startsWith("app")→true

Solution Approach

Build a Trie with TrieNode containing children[26] and isEnd flag.

  • TrieNode: children[26], boolean isEnd.
  • insert: traverse/create nodes char by char, mark last as isEnd.
  • search: traverse nodes; return true only if last node has isEnd=true.
  • startsWith: traverse nodes; return true if all chars found.
Java
class Trie {
    private TrieNode root = new TrieNode();

    public void insert(String word) {
        TrieNode node = root;
        for (char c : word.toCharArray()) {
            if (node.children[c-'a'] == null) node.children[c-'a'] = new TrieNode();
            node = node.children[c-'a'];
        }
        node.isEnd = true;
    }
    public boolean search(String word) {
        TrieNode node = root;
        for (char c : word.toCharArray()) {
            if (node.children[c-'a'] == null) return false;
            node = node.children[c-'a'];
        }
        return node.isEnd;
    }
    public boolean startsWith(String prefix) {
        TrieNode node = root;
        for (char c : prefix.toCharArray()) {
            if (node.children[c-'a'] == null) return false;
            node = node.children[c-'a'];
        }
        return true;
    }
    private static class TrieNode {
        TrieNode[] children = new TrieNode[26];
        boolean isEnd;
    }
}
  • Time Complexity: insert/search/startsWith: O(L) where L=word length
  • Space Complexity: O(N*L) total nodes