When to Use

Look for these signals in the problem:

• "Prefix" search or autocomplete: Trie finds all words with given prefix efficiently.
• "Word dictionary" with insert/search: Trie handles both operations in $O(L)$ time.
• "Wildcard" matching (like '.'): Trie enables DFS exploration at wildcard positions.
• "Word search" in grid with dictionary: Trie prunes paths not matching any word prefix.
• "Longest common prefix": Traverse trie until branching to find shared prefix. Uses $O(\text{total characters})$ space.